# First Assessment of the Thryssa vitrirostris (Engraulidae) Beach Seine Fishery in Northeastern Mozambique

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## Abstract

**:**

_{∞}) and growth rate (K) were 25.1 cm (standard length) and 0.41 per year (standard length), respectively. Two proxy recruitment peaks were found: the first peak of recruitment occurs from April to July, and the second recruitment peak from September to October. The total estimated mortality rates (Z), natural mortality (M), and fishing mortality (F) were Z = 1.31, M = 0.92, and F = 0.39. For the beach seine gear, the size at first catch of T. vitrirostris was Lc

_{25}= 4.43 cm. The 50% retention size of the catch was Lc

_{50}= 5.39 cm. The retention probability analyses revealed a large rate of juvenile fishing mortality (54.2%). The estimated exploitation rate (0.30) was below the maximum exploitation rate (0.48), and above the optimal sustainable exploitation rate (E

_{50}= 0.28), evidencing a sustainable fishery. However, under such an exploitation regime, it is advised that a continuously monitoring-survey of T. vitrirostris is maintained. An increase in migration of fishermen has been recently recorded in Pebane, due to its rich fisheries, which can increase the fishing effort and the risk of overexploitation if management measures (such as mesh size increase) are not taken in advance.

## 1. Introduction

## 2. Results

#### 2.1. Growth Parameters

_{∞}) values required to seed the Von Bertalanffy Plot (VBP) estimated by the different techniques are given in Table 1.

_{∞}, used for “seed” in the VBF equation, was estimated in Electronic Length-Frequency Analysis (ELEFAN) using the response surface routine. As given in the Table 1, the estimated (L

_{∞}) was L

_{∞}= 25.1 cm. The instantaneous growth rate (K) was K = 0.41 year

^{−1}(goodness of fit: Rn = 0.15). The monthly von Bertalanffy growth curve of T. vitrirostris was superimposed over the normal length frequency histograms and over the restructured length frequency histograms (Figure 1).

#### 2.2. Recruitment Patterns

#### 2.3. Mortality Parameters

^{−1}(Figure 4). The current fishing exploitation rate estimated (E

_{est.}), derived from the analysis of mortality rates, was E

_{est.}= 0.30. The estimated instantaneous natural mortality (M) coefficient was M = 0.92. The determination of the instantaneous fishing mortality coefficient (F) was F = 0.39.

#### 2.4. Probabilities of Capture

_{25}) and the length at which the probably of 50% (Lc

_{50}) and 75% (Lc

_{75}) of individuals are captured by gear, determined through the cumulative probability of capture, (Figure 5) were the following: Lc

_{25}= 4.43 cm, Lc

_{50}= 5.93 cm, and Lc

_{75}= 6.35 cm. The percentage of juveniles (length-classes < 13 cm) comprised 54.2% of the catches, while adults were 45.2%. No statistically significant differences in yearly percentage in number average, were observed (t-test: t = 0.95; df = 5; p = 0.384) between juveniles and adults.

#### 2.5. Relative Yield per Recruit (Y’/R) and Relative Biomass per Recruit (B’/R)

#### 2.6. Yield Isopleths

_{∞}ratio for the currently estimated exploitation regime (E

_{est.}) was Lc/L

_{∞}= 0.24.

## 3. Discussion

_{∞}= 25.1 cm; K = 0.41) differed from those found in a nearby beach at Zalala (L

_{∞}= 22.26; K = 0.44; [19]), and in grouped data of Sofala bank (L

_{∞}=19; K = 0.66; [6]), with the asymptotic length being markedly higher, while the growth rate was lower. According to [20,21], the asymptotic length (L

_{∞}) was basically influenced by food availability and population density, while the growth rate (K) was a parameter that was dependent on genetic and physiological factors, which can vary according to environmental fluctuations.

_{∞}and K) estimated among studies should not be related to putative environmental causes.

_{∞}estimations. Thus, we believe that the growth parameter estimated in the present work, using continuous monthly surveys, can contribute to enhancing the knowledge of the species biology and support Mozambique fisheries managers. The L

_{∞}value estimated corresponded to an increase in previous maximum L

_{∞}off 2.5 cm in northeastern Mozambique [6,19]. However, comparing to [13], the L

_{∞}herein estimated was 0.7 cm below the L

_{∞}value estimated in the Maputo region (Southeaster Mozambique). According to [21], the asymptotic growth (L

_{∞}) and growth rate (K) parameters were inversely related, which means that the greater the asymptotic growth (L

_{∞}), the lower the growth rate (K), and the higher the growth rate (K), the smaller the asymptotic growth. Overall the L

_{∞}value was higher than in other studies conducted in northeastern Mozambique, while the growth rate value (K) was lower [7,19]. The scientific cruise of [13] was conducted with a pelagic trawl. The range of the length-size in [25] did not include length classes below 8 cm, nor higher than 22 cm. The size of the trawl mesh was not mentioned in the report, although we can assume (as standard in a scientific survey) that a small mesh size was used in the cod end. Thus, it is a little speculative to debate whether the growth parameter differences among studies are due to fishing gear, since gear selectivity and fish size retained are linked. However, environmental conditions were conservative. Therefore, we cannot exclude the probability that gear or fishing technique differences could explain the regional differences in parameter estimations.

_{∞}) or the growth rate (K) [27,33]. This can be related to the fact that, unlike fishing mortality, natural mortality is associated with predation and diseases, two factors that are not related to the age of individuals [33].

_{50}= 5.39 cm. Thus, the mesh size used may be inadvisable to exploit the resource if we consider that the first maturation size is 13 cm, which is much larger than the size of the Lc

_{50}, indicating that it is necessary to adapt the biology of the species (maturity size) to the selectivity of the fishing gear.

_{est.}= 0.30) was below the E

_{max.}value (E

_{max.}= 0.48) but above the optimal exploitation rate value (E

_{50}= 0.28). This means that the exploitation pressure is above the value of E under which the stock has been reduced to 50% of its unexploited biomass. According to [32,33], a fishing stock is considered to be at a sustainable exploitation level when the exploitation rate does not exceed 50% (E

_{50}), the point at which natural mortality (M) and fishing mortality (F) are at equilibrium. Recent studies confirmed significant increases in the number of fishing gear and fishermen into Zambézia Province, Pebane, due to higher fishing yields expected to be obtained in this area [6]. So, some concerns arise from the future of T. vitrirostris artisanal fishing exploitation regimes in Sofala bank, namely in Pebane, an area where species forms a single demographic population and where the core of the exploitable population is found [6].

_{est.}= 0.3 and Lc/L

_{∞}= 0.24 values, the production rate falls in a exploitation regime that requires some concern on the part of the fishery managers [34], namely, continuously monitoring of the fishery to ensure maintained fishing mortality rate at a steady state. Another concern is the amount of catches under small mesh sizes that can increase the risk of overexploitation [34]. If fishing managers considered maintaining the fishing exploitation rate as it stands (E

_{max.}> E

_{est.}> E

_{50}), an increase in mesh size can be enforced in advance as a precaution management approach aiming directly at reduce fishing mortality. In several parts of the world, small pelagic species under continuous stock assessment and enforced regulations have been reported to be at biological risk, due to high exploitation levels [26,35]. Taken together, and considering the socio-economic importance of T. vitrirostris, the currently exploitation regime might require the implementation of management measures to avoid biomass reduction to unsustainable levels. At the current exploitation stage, it is essential to continue with monitoring surveys and evaluate the risk associated with fishing effort increases as fishing precautionary approaches.

## 4. Materials and Methods

#### 4.1. Study Area

#### 4.2. Data Collection

#### 4.3. Data Analysis

#### 4.3.1. Growth Parameters

_{∞}) was selected based on the Rn value, that measure the goodness of fit (the higher Rn, the better the fitting). The distribution of length frequencies in histograms was adjusted and analyzed, based on the seasonal growth curves of von Bertalanffy, since this technique serves to adjust the observed mean length at age for different combined cohorts.

_{∞}) and growth rate (K) were estimated from the seasonal equation of von Bertalanffy, developed by [41], and later modified by Somers [26,27] as the following:

_{t}—is the length at time t, L

_{∞}—asymptotic length, K—growth rate, t—time, t

_{o}—the age at zero length, C—amplitude of growth oscillations, t

_{s}—the time between birth and onset of the first growth oscillations.

_{t}) on average towards the asymptotic length (L

_{∞}) at an instantaneous growth rate (K) [42]. This means that growth is based on the modal displacement in time sequences of the length samples [31,43]. There are several techniques that are available, that were tested to estimate the L

_{∞}required for the “seed” L

_{∞}in the VBF Equation (1) when the K-Scan procedure is used:

- (i)
- The Powell–Wetherall plot method was used to obtain an initial estimate of (L
_{∞}= 27.11 cm). This method takes into consideration the distribution of length frequency, pooled against the midlength [26,44], allowing for verification of the behavior or variation of the individual sizes in relation to L_{∞}. The mathematical model of this method is described below:L_{middle}– L′ = a + b·L′L_{middle}—is the mean length of all fish (≥L′) and was calculated as:$${\mathrm{L}}_{\mathrm{middle}}=\frac{{\mathrm{L}}_{\infty}-{\mathrm{L}}^{\prime}}{1+\frac{\mathrm{Z}}{\mathrm{K}}}$$L′ is the cutoff length, the length that once reached, the individual has maximum probability of being caught by fishing, L_{∞}= −a/b and Z/K = −(1 + b)/b. - (ii)
- L
_{∞}required for “seed” L_{∞}in VBF Equation (1) was determined based on the maximum length ratio observed (L_{max.}/0.95) in the original length frequency data (maximum observed length-class = 25 cm) according to [45]. - (iii)
- At last, the L
_{∞}value from length frequency data was estimated using the response value routine in ELEFAN. Using the latter approach (iii), and removing the two higher length-classes, that showed an irregular distribution (frequency occurrence very low), we were able to get a good fit for the VBF growth Equation (1) and for both mortality rates and catch plot curves.

#### 4.3.2. Growth Parameters (L_{∞}) and Model Selection

_{∞}was estimated considering all length-classes, from 4 to 25 cm. The L

_{∞}values estimated using (i), (ii), and (iii) allowed for a good adjustment of the VBF growth Equation (1). L

_{∞}values estimated using in (i) and (ii) with all length-classes did not perform well for subsequent analyses, namely, the length converted-catch curves and the mortality rates estimations. This can be related with the low frequency of occurrence and the low number of individuals, for large length-class size distribution [39,43]. In fact, data exploitation analyses reveal that the two higher-length classes (24 and 25 cm) showed an irregular distribution (frequency occurrence that was very low). By removing the 24 and 25 cm length classes, it was possible to achieve a good adjustment of the VBF growth equation (Rn and r-coefficient of correlation), and both mortality rates and catch plot goodness of fit in all analyses. Taken together, the best model or the best estimated L

_{∞}value (statistical fit) necessary to seed the VBF equation to carry out subsequent analyses, was the one estimated by the response value routine in ELEFAN excluding 24–25 length-classes (Table 1).

#### 4.3.3. Recruitment Patterns

#### 4.3.4. Mortality Parameters

#### Natural Mortality

_{∞}+ 0.6543 log K + 0.4634 log T (°C)

_{∞}and K are the parameters obtained from ELEFAN I, and T (°C) is the mean yearly surface water temperature in the study area (average SST = 21 °C, www.fishbase.org studies; [11]). This model takes into account the instantaneous mortality rate, and is very suitable model for small pelagic fish [27,43].

#### Total Mortality

_{i}is the initial number of fish in length class i, and N

_{t}is the number at time t (t is the age corresponding to the mid-length class i). Z with seasonality was then computed from the regression equation. The instantaneous fishing mortality (F) coefficient was estimated as:

_{∞}, K, and t

_{o}, and also the identification of the smallest length class that is recruited (L′). The best model can be selected based on statistical values of length-converted catch curves regression analyses. By selecting or deselecting the first and last data observed points, regression analyses are computed automatically, and based on the goodness of fit, the best model can be selected. The extrapolated or selected points will be used to approximate the probability of capture of length-converted catch curves (for details see: [48]).

#### 4.4. Probabilities of Capture

_{25}, Lc

_{50}and Lc

_{75}). The size values for 25, 50, and 75% of capture (Lc

_{25}, Lc

_{50}, and Lc

_{75}) were estimated from the logistic curve adjusted to the cumulative probability of capture of T. vitrirostris [43].

#### 4.5. Relative Yield per Recruit (Y’/R) and Relative Biomass per Recruit (B’/R)

_{∞})—the growth to be completed after entry into the exploitation phase;

_{10}), the exploitation rate at which the marginal increase of the relative yield-per-recruit is 1/10th of its value at E = ¨0; (ii) the optimum sustainable yield (E

_{50}), or the yield for the reduction of 50% of the stock or value of E under which the stock has been reduced to 50% of its unexploited biomass, and (iii) the maximum sustainable yield (MSY or E

_{max.}), which is the exploitation rate that produces the maximum yield.

#### 4.6. Yield Isopleths

_{max.}) and the critical length ratio Lc/L

_{∞}[42].

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Monthly von Bertalanffy growth curve of Thryssa vitrirostris superimposed over the normal length frequency histograms (

**A**) and superimposed over the restructured length frequency (LF) histograms (

**B**). LF (

**A**) allows for the covering of the life span of several cohorts, and reconstructs an average cohort, while reconstructed LF (

**B**) allows for study of temporal fluctuations of the recruitment.

**Figure 3.**Monthly recruitment pattern of Thryssa vitrirostris by the year (

**A**), monthly percentage of 4 cm of length class registered along the years (

**B**), monthly percentage of 5 cm of length class registered along the years (

**C**).

**Figure 4.**Length converted catch curves applied to length frequencies data of Thryssa vitrirostris. The slope of the right descending arm (black dots) of the curve allow the estimation of total mortality (Z). White dots (expected values) are the expected number of fish, and grey dots (empirical data) the numbers of fish actually sampled (observed).

**Figure 5.**Probability of capture to each size class of Thryssa vitrirostris obtained from the length-converted probability catch curve (

**A**) and percentage of juveniles (length-class < 13 cm) and adults (

**B**) for the six year period (2009–2014).

**Figure 6.**Beverton and Holt’s relative yield per recruit and average biomass per recruit models for Thryssa vitrirostris (

**A**): E

_{10}—green line; E

_{50}—optimum sustainable yield, redline, and the (E

_{max.}—maximum sustainable yield, yellow line. Yield isopleths plot (

**B**) with exploitation rate variability across the critical length ratio, Lc/L

_{∞}.

**Table 1.**Summary of the stock assessment parameters for all models tested: L

_{∞}= Asymptotic length (cm); K = Growth rate; Rn = Goodness of fit; Z/K = The ratio of total mortality to growth rate; Z = Total mortality; M = Natural mortality; F = Fishing mortality (cm); E

_{est.}= Estimated exploitation rate; E

_{max.}= Maximum Sustainable Exploitation rate; M/K = ratio of natural mortality to growth rate; Lc/L

_{∞}= ratio of middle-length to asymptotic length; r = coefficient of correlation.

Method | L_{∞}(cm) | K (cm) | Rn | Z/K | Z (year ^{−}^{1}) | M (year ^{−}^{1}) | F (year ^{−}^{1}) | E_{est.}(year ^{−}^{1}) | E_{max.}(year ^{−}^{1}) | M/K | Lc/L_{∞} | r |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Powell Wetheral (4–25 cm) | 27.11 | 0.34 | 0.17 | 1.14 | 0.64 | 0.79 | −0.15 | −0.24 | 0.41 | 2.30 | 0.15 | −0.95 |

Powell Wetheral (4–23 cm) | 21.71 | 2.20 | 0.12 | 3.16 | 5.12 | 2.87 | 2.25 | 0.44 | 0.48 | 1.30 | 0.34 | −0.95 |

Maximum length (4–25 cm) | 26.25 | 0.37 | 0.16 | 7.00 | 0.62 | 0.85 | −0.23 | −0.37 | 0.42 | 2.30 | 0.16 | −0.72 |

Maximum length (4–23 cm) | 24.15 | 0.45 | 0.14 | 3.16 | 1.12 | 0.99 | 0.14 | 0.12 | 0.48 | 2.20 | 0.24 | −0.95 |

Response Surface (4–25 cm) | 28.20 | 0.31 | 0.17 | 7.00 | 0.65 | 0.74 | −0.09 | −0.14 | 0.42 | 0.03 | 0.15 | −0.72 |

Response Surface (4–23 cm) | 25.10 | 0.41 | 0.15 | 3.16 | 1.31 | 0.92 | 0.39 | 0.30 | 0.48 | 2.20 | 0.24 | −0.95 |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Manuessa, B.; Morais, E.; Cerveira Borges, T.; Teodósio, M.A.; Leitão, F. First Assessment of the *Thryssa vitrirostris* (Engraulidae) Beach Seine Fishery in Northeastern Mozambique. *J* **2018**, *1*, 116-132.
https://doi.org/10.3390/j1010012

**AMA Style**

Manuessa B, Morais E, Cerveira Borges T, Teodósio MA, Leitão F. First Assessment of the *Thryssa vitrirostris* (Engraulidae) Beach Seine Fishery in Northeastern Mozambique. *J*. 2018; 1(1):116-132.
https://doi.org/10.3390/j1010012

**Chicago/Turabian Style**

Manuessa, Bonifácio, Eurico Morais, Teresa Cerveira Borges, Maria Alexandra Teodósio, and Francisco Leitão. 2018. "First Assessment of the *Thryssa vitrirostris* (Engraulidae) Beach Seine Fishery in Northeastern Mozambique" *J* 1, no. 1: 116-132.
https://doi.org/10.3390/j1010012