Open Access
Pré-publication électronique
Dans une revue
Hydroécol. Appl.
DOI https://doi.org/10.1051/hydro/2020001
Publié en ligne 10 avril 2020

© EDF 2020

1 Introduction

Tilapia (including all species) is the second most important group of farmed fish after carps, and the most widely grown of any farmed fish in the world (FAO, 2009). The most desirable Nile tilapia (Oreochromis niloticus (Linnaeus, 1758); [Perciformes: Cichlidae]), because of its advantageous biological traits (feeding habits and fast growth) was deliberately introduced in tropical countries during the 1960s up to the 1980s, promoted by international aid and development agencies (FAO, 2009). Consequently this alien fish originating from East Africa, colonized natural freshwater systems and is currently well established in 75 countries (Africa: 16, Asia: 25, Europe: 6, North America: 18, Oceania: 3, South America: 7; (FishBase, 2018).

This species is invasive, and became the dominant species in terms of density and biomass in various tropical colonized areas (Zambrano et al., 2006; Weyl, 2008; Zengeya et al., 2013). In an ecosystem, Nile tilapia population can affect the nutrient cycling (e.g. indirectly contribute to the occurrence of cyanobacteria blooms and water eutrophication (Figueredo and Giani, 2005), trigger the extinction of native and endemic species (Canonico et al., 2005; Casal, 2006) and probably affect other ecological processes (Canonico et al., 2005; Cucherousset and Olden, 2011). So in freshwater tropical colonized areas, the Nile tilapia is now the main fishing resource of protein for human population, and eventually provides food security (FAO, 2009). An adequate fishing management is then required to avoid overexploitation and preserve sustainable fisheries. In this context, knowledge on the key biological parameters (growth, length, mortality) has to be investigated to better understand the local population biology and then to better develop sustainable fisheries.

One of the most popular methods for estimating biological parameters is to use length frequency data that are fast and cheap to gather (Sparre, 1998) or the length-at-age analysis based on scales, opercular bones or otoliths age readings. Both analyses can be performed with the FAO-ICLARM (Fish Center) Stock Assessment Tool (FiSAT II) which is broadly used for tropical fish stock assessment (> 1700 citations) and contains both routines for the analyses (Gayanilo et al., 2005). In this paper the widely used ’length frequency distribution analysis’ was compared to the parameters obtained with ’analysis of length-at-age’ from fish aged with otoliths using FiSAT II. These three methods are the most common in the abundant O. niloticus literature (see ref. of Tab. 1).

The study case population is from Lao People’s Democratic Republic, in the second-largest reservoir in the country, the Nam Theun 2 (NT2). This reservoir was impounded in April 2008 and after a classical trophic upsurge, the system seems to have reached ecological stabilization in 2011 (Cottet et al., 2016). The Nile tilapia represents the major part of the catch by the local fishery and is of significant economic importance (52% of relative biomass contribution of total catch, (Cottet and Visser, 2017). Biological parameters of this fish species constitute essential data to assess population dynamic and the exploitation level for fisheries management. This is especially needed in this oligotrophic reservoir with relatively low fish density and where the fishing activity settled from less than ten years.

Objectives of this study are to provide: (1) First analyses of detailed data on a tilapia population in Lao PDR; (2) comparison of popular bio parameter assessment methods in a context of improving fisheries management and furthermore; (3) comparison of other world tilapia population data.

Table 1

Biological parameters from three methods (2LFA and 1otoliths) of the stock O. niloticus stock using FiSAT II. The parameters are: predicted asymptotic length (L∞, Standard (SL) and Total Length (TL)), growth coefficient K, growth performance index φ’, and hypothetical length at which age is zero t0, the exploitation rate E, total coefficient of mortality Z, natural mortality M, and fishing mortality F. ID=insufficient data.

Paramètres biologiques pour trois méthode (2 LFA et 1 otolithe) du stock d’O.niloticus via FiSAT II. Les paramètres sont : longueur asymptotique prédite (L∞, standard (SL) et longueur totale (TL)), coefficient de croissance K, indice de performance de croissance φ’ et longueur hypothétique pour laquelle l’âge est égal à zéro t0, le taux d’exploitation E, coefficient total de mortalité Z, mortalité naturelle M et mortalité par pêche F. ID = données insuffisantes.

2 Materials and methods

2.1 Study site

The Nam Theun 2 (NT2) hydropower reservoir is located in the center of Lao PDR (Khammouane Province) on two sub catchments of the Mekong River (Descloux et al., 2016). The reservoir impoundment started in April 2008 and commercial operation began in April 2010. The reservoir has a total volume of 3.9 billion m3, covering an area of 489 km2 at full supply level and 86 km2 at the (theoretical) minimum operating level. The project design led to divert the waters from the Nam Theun watershed to the XeBangfai watershed (Descloux et al., 2016). A total of 33 fish species belonging to 10 families were identified in the reservoir (between 2008 and 2013) (Cottet et al., 2016). Cyprinidae dominated fish populations with a total of 24 species. A total number of 2182 households living was estimated in 16 hamlets along the reservoir in 2014; 85% of them are active in fishing activities with a fluctuating average of [2.5–8 kg HouseHold−1 day−1] (Cottet and Visser, 2017).

2.2 Fish sampling

Data were collected with two sampling methods: fish landing and scientific experimental fishing.

Each of the three villages of Done, Nongbouakam and Nakai Tai (stars of the Fig. 1), was sampled for five consecutive days per month between January to December 2016. Each day, the first ten fishermen reaching the landing site were individually interviewed about their fishing gears (type, mesh size, number of hooks, etc.) and their catches regarding O. niloticus were all recorded. For each fisherman, when the number of individuals was higher than 30, only 30 individuals were randomly selected to be further measured and weighted. A total of 11 820 individuals were recorded for the year, representing about 48% of the yield (mixed techniques used from 246 different fishers, including gillnets with mesh size ranging between 10 and 200 mm, hook lines and set lines).

Second data set was obtained from scientific experimental fishing at five stations (Fig. 1). A total of 95 fish were caught from these experimental monthly gillnet surveys in 2016 (gillnets: 25 × 2 m; mesh size: 10, 15, 20, 25, 30, 35, 40, 50, 60 and 70 mm) representing 24 nights of sampling effort. The gillnet mesh sizes were selected to capture a wide range of fish.

thumbnail Fig. 1

Map of the Nam Theun 2 Reservoir in Lao PDR at its higher level and localization of sampling site by experimental gillnet fishing (black dots) and the three sampled villages by fish landings (black stars).

Carte du réservoir de Nam Theun 2 au Laos à son niveau maximal de remplissage et localisation des sites d’échantillonnage par pêche expérimentale (point noir) et des villages échantillonnés par le suivi des débarquements (étoile noire).

2.3 Ethical statement

The experimental fishing were performed by the Nam Theun 2 Power Company who has the national authorization for fish population monitoring and research in rivers and reservoir by the Lao Government namely the Ministry of Agriculture and Forestry representative on Province level and the Nakai Natural Protected area (namely the Watershed Management Protection Secretariat). This authorization includes the traditional fishing method with gillnets used in this study. This authorization started in 2008 to allow fish population monitoring in the dam area and as an obligation of monitoring between the company and the Government of Laos in the Concession Agreement – Environment and Biodiversity Obligation. The sampling did not catch endangered or protected fish species, the targeted species are exploited species and the experimental fishing followed established guidelines for treatment of animals in research.

2.4 Biological parameters

For both sampling methods, the standard length (SL ± 1 mm) and sex of each individual were recorded. Lengths were assigned to a class size of 10 mm.

For the analysis of length-at-age, 258 Nile tilapia individuals were randomly selected from the fish landing in 2016, and from experimental fishing. Age was determined for each individual by counting annuli on the otoliths after validation of only one annulus formation per year (Tessier et al., 2017).

For the length frequency distribution analysis, data were first obtained from fish landing.

This analysis was performed with the Electronic Length Frequency Analysis (ELEFAN I) package incorporated in FiSAT II (Sparre, 1998; Gayanilo et al., 2005). ELEFAN is a very popular method frequently used in length frequency analysis of fish. It is a non-parametric, ad hoc method, and not dependent on cohort distributions directly. Lengths of each cohort are fixed such as in the von Bertalanffy growth model (1) which makes a strong assumption about growth, with the formula containing the asymptotic length (L∞) or the length the species can reach, the length (L) at age class time t, the instantaneous growth coefficient K and t0 the point at which the von Bertalanffy curve intersects the x-axis: (1)

The growth performance index (φ’), also found as Ф’ or ø’ in literature, is quantified using (Pauly and Munro, 1984) formula (2): (2) with L∞ expressed in Total Length (TL) here, as required in FiSAT II routine for comparability. Total mortality coefficient (Z) is estimated from fish landings and experimental fishing using the length-converted catch curve method (Pauly, 1983) from the pooled length frequency data for the study period (the curve = Z). The natural mortality coefficient (M) is estimated following Pauly’s empirical formula (Pauly, 1980) (3): (3)linking natural mortality with the von Bertalanffy parameters (K and L∞) and the mean annual temperature (T °C) of the surface water in which the fish live (in this case) temperatures of the surface water were recorded for several stations every month by the Environment and Social Division, Environment Department of the Nam Theun 2 Power Company Limited which is the manager of the reservoir. The average surface temperature for 2016 was 26.5 °C. Fishing mortality (F) was computed from the relationship F = Z − M, while the exploitation rate (E) was calculated from (4): (4)

The recruitment model with pulse was obtained by projecting the length frequency data back on the time axis using the growth parameters determined above. The normal distribution of the recruitment model was determined by NORMSEP (Gayanilo et al., 2005).

2.5 Virtual population analysis, fish stock and fishing management

The probabilities of capture are inferred from the length-converted catch curves, by backward extrapolation of the catch curve and comparison of the numbers actually caught with those that “ought” to have been caught (Sparre, 1998; Gayanilo et al., 2005). The standard length for which 25, 50 and 75% of the fish are caught (L25,L50, L75) were estimated with running averages technique (Pauly, 1983) with FiSAT II. A virtual population analysis (VPA) estimated the tilapia stock (biomass, population size (N)) at the NT2 Reservoir from fish landing data and parameters determined above. For the biomass estimation, the standard length-weight relationship for these Tilapia was calculated with (5): (5)

where b = [3.23323–3.27171] and a = [0.01699–0.01897] (IC95), N = 653 fish from 6 to 410 mm; length-weight relationship R2 = 0.9943; (unpub. species results, method based on (Tessier et al., 2017)).

Based on the previous parameters and frequencies, FiSAT II can still be used to predict the relative yield and biomass per recruit (Y’/R and B’/R) of O. niloticus to the fisheries. Plots of Y’/R vs. E (=F/Z) and of B’/R vs. E, from which Emax (exploitation rate which produces maximum yield), E0.1(exploitation rate at which the marginal increase of relative yield-per-recruit is 10% of its unexploited biomass E = 0) and E0.5 (value of E under which the stock has been reduced to 50% of its unexploited biomass) are also computed through the first derivative of the Beverton and Holt function (Beverton and Holt, 1966).These are used as target reference points for best fishing management.

Yield contours are plotted to assess the impact on yield of changes in exploitation rate E and critical length ratio Lc/L. (Lc being mean length of fish at first capture, = L50). The resulting isopleths map can be used to determine the best E and Lc/L∞ to reach the maximal Y’/R and B’/R at the current situation.

Additional length is calculated as additional help fishing management decision: Lopt which is the length where the biomass of an unfished cohort reaches its maximum (Beverton, 1992; Froese et al., 2008) (6): (6)

The standard length at which 50% of Nile tilapias are still immature and the other half starts to be mature was estimated based on the 258 fish measured above (SL ± 1 mm). These lengths were compared with Lc. Maturity stage was described by a 1 or a 0 observing: (1) gonads presence (+ ovule for females = stage 3 for this species) and (+ semen dripping from the gonads for males = stage 4) or (0) no visible gonads.

A logistic regression was fitted in R with the general linear model procedure using the fishR package. The Thompson and Bell yield and stock prediction routine in FiSAT II was used for prediction of harvested fish and biomass changes with an example of management recommendation (Thompson and Bell, 1934). This model combines features of Beverton and Holt’s Y’/R model with those of VPA, which it inverts. The current fishing rate and Lc were compared with increased Lc and Lopt determined bellow only, without modification of F.

2.6 Lao reservoir and worldwide φ’ comparison

Biological parameters from other tilapia populations of various ecological contexts (natural and farmed) have been gathered from available literature and analysed for comparisons with the NT2 tilapia population. According to (Moreau et al., 1986), the growth performance index (φ’) values should be equal or roughly similar between different stocks in a normal distribution. We tested this assumption and hypothesised that this pan-tropical species showed a phenotypic plasticity with a wide spectrum of φ’ irrespectively to continents or water systems, but fish from aquaculture raised in optimal settings should have a better φ’ than in natural and challenging conditions (Pauly et al., 1988). We also checked (with objective (2)) if biological parameter variability could also be due to variability in methodology.

To test the normal distribution of worldwide O. niloticus φ’, Shapiro-Wilk and graphical tests were performed on Table 1 values. These data were collected from published works based on bibliographic search, with the keywords “Oreochromis + niloticus” in Web of Science (from 1970 to August 2018) and from relevant works found in the references sections. Continental effects on populations, and other potential effects such as hydrosystems (reservoir, lake, cage, pond, etc.) were tested with analysis of variances, Kruskal-Wallis and Pairwise comparisons using Wilcoxon rank sum tests. These tests were realized with R software (R Development Core Team, 2011). Only the average was considered for cases with multiple values at a same site.

Because the reported fish lengths were standard length (SL) or total length (TL) in the literature, we established the O. niloticus TL/SL relationship based on 322 fish caught and measured into the NT2 Reservoir (the 258 aged Nile tilapia individuals randomly selected from the fish landing in 2016 +64 not aged). The relationship was (R2 = 0.99): TL = 1.223 × SL, and it allowed recalculate comparable growth performance indices (φ’).

3 Results

3.1 Biological parameters

At the NT2 Reservoir, age of the 258 fish given by otolith readings ranged from 0 to 8 years old (Max. ♀ = 8 years, Max. ♂ = 7 years), with standard length ranging from 31 to 360 mm. The L∞ mean (±SE) was 454.7 ± 123.8 mm (♀ = 317.5 ± 124.1 mm; ♂ = 438.1 ± 284.9 mm).The mean growth coefficient (K) was 0.14 ± 0.06 yr−1 (♀ = 0.23 ± 0.34 yr−1;♂ = 0.14 ± 0.17 yr−1) and the growth performance indices (φ’) was 2.636 (♀ = 2.540; ♂ = 2.604; Tab. 1). For length frequency distribution analyses, data ranged from 65 to 495 mm for fish landing and from 37 to 303 mm for experimental fishing. The L∞ means, (K) and (φ’) were respectively (no SE available) L∞= 525.0 mm, (K) = 0.230 yr−1, (φ’) = 2.977 for fish landing and L∞= 241.5 mm, (K) = 0.140 yr−1, (Tab. 2). (φ’) = 2.085 for experimental fishing.

The mean estimated total mortality coefficient (Z) was 1.41 ± 0.23 year−1, IC95 = [1.33–1.50]; natural mortality coefficient (M) = 0.30 year−1 (at 26.5 °C) fishing mortality coefficient (F) = 1.11 year−1 and exploitation rate (E) = 0.79 from the fish landing data (Fig. 2 left). The experimental fishing did not allow relevant results (e.g. relative age of fish at 22 years old, Z < M, etc.).

The recruitment model showed continuous recruitment of O. niloticus all along the year with slight peaks in January and in March–April (11.0 and 18.4%, respectively).

Table 2

Biological parameters of Nile tilapia (O. niloticus) in various locations from literature. The parameters are: predicted asymptotic length (L∞, Standard and Total Length, italics values are recalculated values), growth coefficient K, growth performance index φ’, and hypothetical length at which age is zero t0, the exploitation rate E, total coefficient of mortality Z, natural mortality M, and fishing mortality F.

Paramètres biologiques du Tilapia du Nil (O. niloticus) dans diverses localités et issus de la littérature. Les paramètres sont : longueur asymptotique prédite (L∞, longueur standard et longueur totale, les valeurs en italique sont des valeurs recalculées), coefficient de croissance K, indice de performance de croissance φ’, longueur hypothétique pour laquelle l’âge est égal à zéro t0, taux d’exploitation E, coefficient de mortalité total Z, naturel mortalité M et mortalité par pêche F.

thumbnail Fig. 2

Length-converted catch curve (left) of O. niloticus (N = 11820) estimating annual instantaneous total mortality (Z) = 1.41 year−1; Natural mortality (at 26.5 °C) (M) = 0.30 year−1; Fishing mortality (F) = 1.11 year−1; Exploitation rate (E) = 0.79. NT2 Reservoir, Lao, PDR (year 2016). Black dots of the left figure were used for Z calculation (i.e. the slope) using least squares line regression (R2 = 0.98). The white dots represent the fish that would have been caught if fully recruited. The yellow dots on the left are the not fully recruited points discarded from the calculation; The ratio: expected and actual catches, gives probability of capture for each class size(right).

À gauche : Courbe des captures converties en tailles d’O. niloticus (N = 11820) estimant la mortalité instantanée annuelle (Z) = 1,41 an−1 ; naturelle (à 26,5 °C) (M) = 0,30 an−1 ; par pêche (F) = 1,11 an−1 ; le taux d’exploitation (E) = 0,79 pour Nam Theun 2 en 2016. Les ronds noirs correspondent aux données utilisées pour calculer Z en utilisant la régression linéaire des moindres carrées (R2 = 0.98). Les ronds blancs représentent les poissons qui auraient été capturés si le recrutement avait été total. Les ronds jaunes sont des données correspondant à un recrutement non total et écartés du calcul. À droite : Le ratio : les captures prévues et les captures réelles donnent la probabilité de capture pour chaque taille de classe.

3.2 Virtual population analysis and fishing management

With current Fish mortality, F of 1.11, the optimal Fopt should be 0.5M = 0.15 and the limit Flim should be ⅓M = 0.10. The mean length at the entry into the fishery at L25, L50, and L75 were 202.43, 210.38 and 219.28 mm respectively (Fig. 2 right). The mean length of fish at first capture (Lc) was equivalent to L50 = 210.38 mm. Immature fish were found up to 360 mm but some mature fish started at 112 mm. The proportion of mature fish was 50% for a standard length of 295 mm (Fig. 3). From the FiSAT II virtual population analysis, the stock of the reservoir was estimated to be 165 tones with about 744 000 Nile tilapias (Fig. 4). The exploitation rate was E = 0.79. This rate was > E10 = 0.508, > E50 = 0.336 and > Emax = 0.594 obtained by relative yield and biomass per recruit (Fig. 5). The yield-per-recruit isopleths diagram of the various length at entry for O. niloticus into the fishery based on different values of E and a constant value of M/K = 1.304 and L∞ = 525 mm showed the discontinued curves indicating the range producing the maximum yield-per-recruit (Fig. 6). The maximum theoretical value of relative yield-per-recruit at the meeting point of the eumetric yield curve (i.e. stable population) with the maximum sustainable yield (MSY) (so-called potential yield-per-recruit) was Y’/R = 0.062 with E = 0.7 and Lc/L∞ = 0.6, that to say Lc = 315 mm. The current level of exploitation (Y’/R = 0.05) is on the right side of the eumetric curve indicating potential overfishing with too many short fish caught (Fig. 6). Accordingly to the figures, a reduction of E (moving to the left) and/or a reduction of the targeted class size (with bigger hooks and larger gill net mesh (moving to the top)), would lead to an increase in relative yield.

Value of Lopt = 365.9 mm is considered as the size of exploitation at which O. niloticus yield attains its maximum potential in this reservoir.

A Thompson and Bell Yield-Stock Prediction with a L50 of 315 mm was compared to the current prediction with L50= 210.38 mm. For the same fishing effort of F = 1.11, but with an increase of mesh size, the yield was near optimal capability and with +125% than the current level. On the other hand, the biomass has more than doubled (207%).

thumbnail Fig. 3

Fitted logistic regression for proportion of mature Nile tilapia (O. niloticus) by standard length with L50 = 295 mm shown in blue (N = 258) in NT2 Reservoir, Lao, PDR. The current mean length of fish at first capture was Lc = 210.38 mm.

Régression logistique ajustée pour la part de Tilapia du nil (O. niloticus) mature selon la longueur standard avec L50 = 295 mm indiqué en pointillé bleu (N = 258) pour Nam Theun 2, Laos. La longueur moyenne actuelle des poissons à la première capture (Lc) était de 210,38 mm.

thumbnail Fig. 4

Length-Structured (TL) Virtual Population Analysis (VPA) of O.niloticus (from fish landings) in NT2 Reservoir, Lao, PDR (year 2016).

Analyse de la structure de taille (TL) d’une population virtuelle (VPA) d’O.niloticus (issus des débarquements de pêche) pour le réservoir de Nam Theun 2, au Laos, en 2016.

thumbnail Fig. 5

Relative yield and biomass per recruit (selection ogive) of O.niloticus. NT2 Reservoir, Lao, PDR (year 2016). E10 = 0.508, E50 = 0.336 and Emax = 0.594 (yellow dotted line). Current E = 0.79.

Rendement relatif et biomasse par recrut (sélection ogive) d’O. niloticus pour le réservoir de Nam Theun 2, au Laos, en 2016. E10 = 0.508, E50 = 0.336 and Emax = 0.594 (ligne pointillée jaune). Actuellement E = 0,79.

thumbnail Fig. 6

3D map of relative yield per recruit (Y’/R) (left), and relative biomass per recruit (B’/R) (right) as a function of relative size at first capture (Lc/L∞) and exploitation rate (E = F/Z Knife-edge with fixed M/K = 1.304. The relative yield and relative biomass per recruit increased with the vertical axis (isopleth color going to orange). For the Y’/R, if the situation point is at the left of the slope (eumetric yield curve) the scenario is in underfishing situation, at the right of the slope: in overfishing. The situation in 2016 in the NT2 Reservoir for Nile tilapia was E = 79 and Lc/L∞ = 0.4. A reduction in exploitation rate and in Lc with mesh size/hooks increase lead to an increase in relative yield (and overall biomass) and would be the fishing management recommendation.

Représentation 3D du rendement relatif par recrue (Y’/R) (à gauche) et de la biomasse relative par recrue (B’/R) (à droite) en fonction de la taille relative à la première capture (Lc/L∞) et du taux d’exploitation (E=F/Z). Tranche d’âge avec M/K fixé à 1,304. Le rendement relatif et la biomasse relative par recrue ont augmenté avec l’axe vertical (la couleur des isoplèdes allant à l’orange). Pour le Y’/R, si le point de situation est à gauche de la pente (courbe de rendement eumétrique), le scénario est en situation de sous-pêche, à droite de la pente : en sur pêche. La situation en 2016 du réservoir de Nam Theun 2 pour le tilapia du Nil était E = 79 et Lc/L∞ = 0,4. Une réduction du taux d’exploitation et de Lc avec une augmentation de la taille du maillage ou recourt aux hameçons conduit à une augmentation du rendement relatif (et de la biomasse globale) et constituerait donc une recommandation de gestion pour cette pêche.

3.3 Lao reservoir and worldwide φ’ comparison

Biological parameters of several Nile tilapia populations from various continents and hydrosystems are summarized in Table 2. The range of parameter values was wide for populations from the different ecosystems (lakes, reservoirs, rivers): L∞ = [SL: 145.5–591.3 cm], K = [0.07–0.70], φ’ = [1.56–3.31]. Globally the φ’ distribution was normal (W = 0.94605, p-value = 0.0001584). The average φ’ was significantly higher in fish farms = 3.387; IC95 = [3.332–3.443] (ponds, cages, tanks, Tab. 3) than in ecosystem 2.735; IC95 = [2.624–2.846] (lakes, reservoirs, rivers) (W = 173, p < 0.0001). There were differences within the ecosystem and aquaculture groups (Wilcoxon rank sum tests with P < 0.05, Fig. 7). For ecosystems, the populations were not different between Africa and Asia (W = 222, p-value = 0.797).

Table 3

Biological parameters of Nile tilapia (O. niloticus) in aquaculture (Pauly et al., 1988). With predicted asymptotic length (L∞, Standard and Total Length), growth coefficient K, and growth performance index φ’.

Paramètres biologiques du Tilapia du Nil (O. niloticus) en aquaculture (Pauly et al., 1988). Les paramètres sont : les longueurs asymptotiques prédite (L∞ pour la longueur standard et la longueur totale), le coefficient de croissance K et l’indice de performance de croissance φ’.

thumbnail Fig. 7

The growth performance indices (φ’) of Nile tilapia (O.niloticus) in various hydrosystems. Groups significantly different are represented by letters (Wilcoxon rank sum tests, P < 0.05). Below each box, the effective of bibliographic resources indicates the number of used. N are indicated below the boxes. The star represents the NT2 Reservoir, Lao PDR.

Les indices de performance de croissance (φ’) du Tilapia du Nil (O.niloticus) dans divers hydro-systèmes. Les groupes significativement différents sont identifiés par une lettre (test de Wilcoxon, P < 0,05. L’étoile représente Nam Theun 2 au Laos.

4 Discussion

The NT2 Reservoir was impounded in 2008 and tends to reach an ecological equilibrium, despite significant exploitation of the ichtyo resources (Cottet et al., 2016; Cottet and Visser, 2017). More than 2000 households rely on the O. niloticus for food. representing half of their fisheries. Consequently, knowledge on biological parameters of O. niloticus is useful for fishing management in this recent Asian reservoir and more largely for the South East Asian region. Globally, several methods exist to help fish stock management, most of them usable in various routines with FiSAT II (Sparre, 1998). This study showed that the results can be slightly different between length-at-age analyses versus length frequency distribution growth analyses, with the same fish stock, sampled at the same year. The sampling method can affect biological parameters results and thus potentially changing other parameters such as mortality M or even Beverton and Holt Y/R analyses (Knife-edge) which is calculated mainly with K and L∞.

Otolith readings are a robust method but the lower sample size of this study prevented recording fish of extreme large sizes (for L∞ determination). For instance, a 8 years old female was aged in this study, which is the oldest age published for Nile tilapia in the literature (FishBase, 2018), but had a SL of only 280 mm (in the literature). Thus indicating a probable margin of more growing. The L∞ is = 454 versus 525 mm for the length at age and the length frequency analyses respectively. However, K was 0.14 for males and for all sex pooled, but 0.23 for females only, equal to 0.23 found from the length frequency analyses. These results complicated the inter and intra-population comparisons regularly found in literature (Tab. 2). Furthermore, site comparisons have to be cautious because some parameters based on experimental fishing with are from low sample size or short period of sampling. However, in this study case, insufficient sampling showed irrelevant results for this type of analysis despite costly sampling efforts (24 nights, 3 staff/boat for this protocol). The main conclusion for fish landings or experimental sampling choice of protocol is that bulky data based on fishermen are more robust even if the catches are not standardized with a fishing protocol, because of the high data number reaching representativeness. Furthermore, this method is logistically easier. This method is thus recommended according to the scientific questions and needs. This example recalls best practices for using the ELEFAN approach (Brey and Pauly, 1986; Pauly and Morgan, 1987; Taylor and Mildenberger, 2017): the length frequency data should (i) contain relatively high counts that are representative of the length distribution of the population or catches; (ii) be sampled at regular intervals throughout the year with the same sampling effort and (iii) cover a substantial degree of small class sizes, as their relatively higher numbers and faster growth will aid in the fitting of the growth curve. A general rule-of-thumb could be that the smallest class size should start at least 25% of L∞. Our experimental fishing study case failed for condition (i).

This finding highlights the need to compare the biological parameters of a fish population with cautious. Indeed, various methods seem to provide slightly different results. The species range for K is [0.14–0.41 years−1] on Fishbase (FishBase, 2018), but seems to be wider on Table 1. φ’ can be used to compare populations as long as similar units and definitions are used (see routine’s requests in FiSAT II) and cautions have to be taken for ichthyologists for comparisons using external φ’ data. However, the meaningful low growth performance indices φ’ can be noticed for populations suffering challenging environment (e.g. oligotrophic waters, low temperature, etc. and high φ’ for population under relatively severe predation/fishing pressure (Amarasinghe, 2002; Njiru et al., 2007; Moreau et al., 2008; Costa Novaes and Carvalho, 2012; Yongo and Outa, 2016).

Based on these data, O.niloticus of NT2 Reservoir seems to be overfished. The current E is 0.79 which is superior to Emax = 0.594, leading to an over exploitation of the population with reduced biomass, density and ultimately to reduced yield per recruit relatively to its optimal potential (0.05 instead of 0.062). Reducing the fishing mortality from 1.11 to 0.45 would increase the population biomass by 195% and thus the yield by 114%. Furthermore, the fish start to be caught at 210 mm through the current gillnet mesh size. Many of these fish did not reproduced more than once or are even immature and will never participate to the population renewal. Using bigger mesh size would lead to an increase in the yield. The social, economic and technical implications of changes in current fishing practices must not be overlooked. A dedicated feasibility study should probably be done and a pedagogic and technical support should be provided to local populations. Other ecological effects of the O. niloticus deserve to be studied.

Acknowledgments

This work was funded by the EDF-CIH (Centre d’Ingénierie Hydraulique – France), by the Nam Theun 2 Power Company (NTPC – Lao PDR) through a project led by UMR CARRTEL (INRA – USMB, France). The fieldwork was performed thanks to the Nam Theun 2 Power Company, whose shareholders are Électricité de France, Lao Holding State Enterprise and Electricity Generating Public Company Limited of Thailand. The authors would like to thank all villagers who participated in the fish landing survey, the staff of the NTPC, especially the Environment Department, including Khampong THAN-Onkeo, Sengthien Vongvilaysuk, Seesouk XAYABOUNPHENG, and Sengsavanh PHOMPHILATH, for their technical and human support in the laboratory in Lao PDR. We would also like to thank Michel COLON and Jean-Christophe HUSTACHE of the UMR CARRTEL for their help in the field and the laboratory in Lao PDR.

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All Tables

Table 1

Biological parameters from three methods (2LFA and 1otoliths) of the stock O. niloticus stock using FiSAT II. The parameters are: predicted asymptotic length (L∞, Standard (SL) and Total Length (TL)), growth coefficient K, growth performance index φ’, and hypothetical length at which age is zero t0, the exploitation rate E, total coefficient of mortality Z, natural mortality M, and fishing mortality F. ID=insufficient data.

Paramètres biologiques pour trois méthode (2 LFA et 1 otolithe) du stock d’O.niloticus via FiSAT II. Les paramètres sont : longueur asymptotique prédite (L∞, standard (SL) et longueur totale (TL)), coefficient de croissance K, indice de performance de croissance φ’ et longueur hypothétique pour laquelle l’âge est égal à zéro t0, le taux d’exploitation E, coefficient total de mortalité Z, mortalité naturelle M et mortalité par pêche F. ID = données insuffisantes.

Table 2

Biological parameters of Nile tilapia (O. niloticus) in various locations from literature. The parameters are: predicted asymptotic length (L∞, Standard and Total Length, italics values are recalculated values), growth coefficient K, growth performance index φ’, and hypothetical length at which age is zero t0, the exploitation rate E, total coefficient of mortality Z, natural mortality M, and fishing mortality F.

Paramètres biologiques du Tilapia du Nil (O. niloticus) dans diverses localités et issus de la littérature. Les paramètres sont : longueur asymptotique prédite (L∞, longueur standard et longueur totale, les valeurs en italique sont des valeurs recalculées), coefficient de croissance K, indice de performance de croissance φ’, longueur hypothétique pour laquelle l’âge est égal à zéro t0, taux d’exploitation E, coefficient de mortalité total Z, naturel mortalité M et mortalité par pêche F.

Table 3

Biological parameters of Nile tilapia (O. niloticus) in aquaculture (Pauly et al., 1988). With predicted asymptotic length (L∞, Standard and Total Length), growth coefficient K, and growth performance index φ’.

Paramètres biologiques du Tilapia du Nil (O. niloticus) en aquaculture (Pauly et al., 1988). Les paramètres sont : les longueurs asymptotiques prédite (L∞ pour la longueur standard et la longueur totale), le coefficient de croissance K et l’indice de performance de croissance φ’.

All Figures

thumbnail Fig. 1

Map of the Nam Theun 2 Reservoir in Lao PDR at its higher level and localization of sampling site by experimental gillnet fishing (black dots) and the three sampled villages by fish landings (black stars).

Carte du réservoir de Nam Theun 2 au Laos à son niveau maximal de remplissage et localisation des sites d’échantillonnage par pêche expérimentale (point noir) et des villages échantillonnés par le suivi des débarquements (étoile noire).

In the text
thumbnail Fig. 2

Length-converted catch curve (left) of O. niloticus (N = 11820) estimating annual instantaneous total mortality (Z) = 1.41 year−1; Natural mortality (at 26.5 °C) (M) = 0.30 year−1; Fishing mortality (F) = 1.11 year−1; Exploitation rate (E) = 0.79. NT2 Reservoir, Lao, PDR (year 2016). Black dots of the left figure were used for Z calculation (i.e. the slope) using least squares line regression (R2 = 0.98). The white dots represent the fish that would have been caught if fully recruited. The yellow dots on the left are the not fully recruited points discarded from the calculation; The ratio: expected and actual catches, gives probability of capture for each class size(right).

À gauche : Courbe des captures converties en tailles d’O. niloticus (N = 11820) estimant la mortalité instantanée annuelle (Z) = 1,41 an−1 ; naturelle (à 26,5 °C) (M) = 0,30 an−1 ; par pêche (F) = 1,11 an−1 ; le taux d’exploitation (E) = 0,79 pour Nam Theun 2 en 2016. Les ronds noirs correspondent aux données utilisées pour calculer Z en utilisant la régression linéaire des moindres carrées (R2 = 0.98). Les ronds blancs représentent les poissons qui auraient été capturés si le recrutement avait été total. Les ronds jaunes sont des données correspondant à un recrutement non total et écartés du calcul. À droite : Le ratio : les captures prévues et les captures réelles donnent la probabilité de capture pour chaque taille de classe.

In the text
thumbnail Fig. 3

Fitted logistic regression for proportion of mature Nile tilapia (O. niloticus) by standard length with L50 = 295 mm shown in blue (N = 258) in NT2 Reservoir, Lao, PDR. The current mean length of fish at first capture was Lc = 210.38 mm.

Régression logistique ajustée pour la part de Tilapia du nil (O. niloticus) mature selon la longueur standard avec L50 = 295 mm indiqué en pointillé bleu (N = 258) pour Nam Theun 2, Laos. La longueur moyenne actuelle des poissons à la première capture (Lc) était de 210,38 mm.

In the text
thumbnail Fig. 4

Length-Structured (TL) Virtual Population Analysis (VPA) of O.niloticus (from fish landings) in NT2 Reservoir, Lao, PDR (year 2016).

Analyse de la structure de taille (TL) d’une population virtuelle (VPA) d’O.niloticus (issus des débarquements de pêche) pour le réservoir de Nam Theun 2, au Laos, en 2016.

In the text
thumbnail Fig. 5

Relative yield and biomass per recruit (selection ogive) of O.niloticus. NT2 Reservoir, Lao, PDR (year 2016). E10 = 0.508, E50 = 0.336 and Emax = 0.594 (yellow dotted line). Current E = 0.79.

Rendement relatif et biomasse par recrut (sélection ogive) d’O. niloticus pour le réservoir de Nam Theun 2, au Laos, en 2016. E10 = 0.508, E50 = 0.336 and Emax = 0.594 (ligne pointillée jaune). Actuellement E = 0,79.

In the text
thumbnail Fig. 6

3D map of relative yield per recruit (Y’/R) (left), and relative biomass per recruit (B’/R) (right) as a function of relative size at first capture (Lc/L∞) and exploitation rate (E = F/Z Knife-edge with fixed M/K = 1.304. The relative yield and relative biomass per recruit increased with the vertical axis (isopleth color going to orange). For the Y’/R, if the situation point is at the left of the slope (eumetric yield curve) the scenario is in underfishing situation, at the right of the slope: in overfishing. The situation in 2016 in the NT2 Reservoir for Nile tilapia was E = 79 and Lc/L∞ = 0.4. A reduction in exploitation rate and in Lc with mesh size/hooks increase lead to an increase in relative yield (and overall biomass) and would be the fishing management recommendation.

Représentation 3D du rendement relatif par recrue (Y’/R) (à gauche) et de la biomasse relative par recrue (B’/R) (à droite) en fonction de la taille relative à la première capture (Lc/L∞) et du taux d’exploitation (E=F/Z). Tranche d’âge avec M/K fixé à 1,304. Le rendement relatif et la biomasse relative par recrue ont augmenté avec l’axe vertical (la couleur des isoplèdes allant à l’orange). Pour le Y’/R, si le point de situation est à gauche de la pente (courbe de rendement eumétrique), le scénario est en situation de sous-pêche, à droite de la pente : en sur pêche. La situation en 2016 du réservoir de Nam Theun 2 pour le tilapia du Nil était E = 79 et Lc/L∞ = 0,4. Une réduction du taux d’exploitation et de Lc avec une augmentation de la taille du maillage ou recourt aux hameçons conduit à une augmentation du rendement relatif (et de la biomasse globale) et constituerait donc une recommandation de gestion pour cette pêche.

In the text
thumbnail Fig. 7

The growth performance indices (φ’) of Nile tilapia (O.niloticus) in various hydrosystems. Groups significantly different are represented by letters (Wilcoxon rank sum tests, P < 0.05). Below each box, the effective of bibliographic resources indicates the number of used. N are indicated below the boxes. The star represents the NT2 Reservoir, Lao PDR.

Les indices de performance de croissance (φ’) du Tilapia du Nil (O.niloticus) dans divers hydro-systèmes. Les groupes significativement différents sont identifiés par une lettre (test de Wilcoxon, P < 0,05. L’étoile représente Nam Theun 2 au Laos.

In the text

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