INSTITUTO NACIONAL DE INVESTIGAÇÃO PESQUEIRA
REVISTA MOÇAMBICANA DE INVESTIGAÇÃO
PESQUEIRA
RIP No. 32, pp 15-29, 2012
Depletion Experiments to Assess Trawl Efficiency Components of
Catchability for Penaeid Shrimps in the Multi-Species Commercial
Trawl Fishery of Sofala Bank, Mozambique
by
Atanásio Brito
a,c,*, David Die
b, Larry Robinson
aand James Penn
da
Florida A&M University, Environmental Sciences Institute, FL 32307 USA
b
University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami FL 33149 USA
c
Instituto Nacional de Investigação Pesqueira, P.O. Box 4603, Maputo, Mozambique
d
Index
Abstract……….17
Resumo………...17
1. Introduction ………...19
2. Materials and Methods………..20
2.1. The study area……….20
2.2. Depletion Experiment……….20
2.3. Catch sampling and data recorded………..21
2.4. Data processing and statistical analysis………..23
3. Results………23
3.1. Depletion rates and trawl-efficiency………...23
3.2. Effect of environmental factors on trawl efficiency………...25
4. Discussion………..26
5.Acknowledgments………..27
Abstract
Where the efficiency of fishing gear is not known, fisheries assessments often use the inaccurate but unavoidable assumption that catch per unit of effort (CPUE) is a direct reflection of abundance. An understanding of fishing gear efficiency is therefore necessary to ensure the accuracy of biomass estimation based on trawl sampling data. This study attempts to estimate trawl efficiency for two important penaeid shrimp species taken in the Sofala Bank shrimp fishery in Mozambique. The trawl efficiency for the two nocturnally active shrimp species*, Penaeus latisulcatus (western king prawn) and Penaeus japonicus (kuruma prawn) was assessed using the Leslie-DeLury method which involved repeated trawling of an offshore experimental area during three nights. The linear relationship between abundance (number/trawl) and cumulative catch fitted reasonably well (R2 > 0.8) for both species, and indicated that the standard trawls being used by the industrial vessels had efficiencies of 23% for P. latisulcatus and 24% for P.
japonicus at that time. A GLM to test the effect of a number of environmental variables (tide speed, tide
direction, wind speed, time of trawling) and the effect of fishing gear on the abundance indices indicated that none of the variables were significant for P. latisulcatus. However, P. japonicus catch rates were negatively affected by the fishing gear (cumulative catch) and by trawling at dawn. These trawl efficiency experiments suggest that there can be significant variability in the vulnerability of the nocturnal shrimp species to the trawl gear depending on time of day and that CPUE represents a small proportion of the biomass actually present. These variations in vulnerability to the trawl gear, which affect the catchability of these species, needs to be taken into account when analyzing trawl survey data and undertaking stock assessments generally.
Keywords: Leslie-DeLury depletion; Sofala Bank; Mozambique; Trawl efficiency; Penaeidae
* In this paper, the penaeid shrimp species names used are according to Holthuis (1980), reflecting a re-assessment of the genus Penaeus nomenclature by Flegel (2007).
Resumo
Quando a eficiência de captura de uma arte de pesca é desconhecida, o trabalho de avaliação de pescarias socorre-se da suposição imprecisa mas útil de que a captura por unidade de esforço (CPUE) é reflexo directo da abundância. É necessário um entendimento da eficiência do arrasto para maximizar a precisão de estimação de biomassas baseado em dados de amostragem do arrasto. Este estudo tenta determinar a eficiência de captura para duas espécies de camarões peneídeos pescadas no Banco de Sofala, Moçambique. Foi avaliada a eficiência da pesca por arrasto das duas espécies nocturnas, Penaeus
latisulcatus (camarão real/marfil) e Penaeus japonicus (camarão flor/tigre) através do método de
Leslie-DeLury que consiste no arrasto repetitivo de uma área localizada no mar durante três noites. A relação linear entre a abundância (número/arrasto) e a captura acumulada ajustou-se razoavelmente bem ao modelo (R2 > 0,8) para ambas espécies, e indicou que os arrastos comerciais da frota industrial tem eficiências de 23% para P. latisulcatus e 24% para P. japonicus. Um modelo linear generalizado (GLM) para testar o efeito de um número de variáveis ambientais (velocidade da maré, direcção da maré, velocidade do vento, hora do arrasto) e o efeito da arte de pesca nos índices de abundância indicou que uma das variáveis foi significativa para P. latisulcatus. Porém, os rendimentos de P. japonicus foram negativamente afectados pelo arrasto (captura acumulada) e pela pesca durante o amanhecer. Estas experiências de eficiência do arrasto sugerem que pode haver uma variabilidade significativa na vulnerabilidade destas espécies nocturnas ao arrasto dependendo do período do dia e que a CPUE representa uma pequena proporção da biomassa presente no local. Estas variações na vulnerabilidade de pesca da espécie ao arrasto, que afecta a sua capturabilidade, precisa de ser tomada em conta quando analisados os dados de cruzeiro de investigação e ao realizar avaliações gerais destes recursos pesqueiros.
Palavras chave: Deplecção de Leslie-DeLury; Banco de Sofala; Moçambique; Eficiência do arrasto;
Penaeidae
* Neste trabalho, os nomes dos camarões peneídeos usados estão de acordo com Holthuis (1980), reflectindo a re-avaliação da nomenclatura do género Penaeus por Flegel (2007).
1. Introduction
The catches of penaeid shrimps are dependent upon complex interactions between the target species, the fishing gear and a suite of biotic and abiotic factors at different life stages. The impact of these factors on the dynamics of penaeid shrimp stocks is generally reflected in temporal and spatial variations in distribution and abundance that ultimately impact their vulnerability to capture by commercial trawling. The relationship between resource abundance and the catch per unit of fishing effort is known as catchability (Arreguin-Sanchez, 1996) and can be heavily influenced by the efficiency of the fishing gear. As noted by Penn (1984), in practice, catchability is quantified for a particular stock as a “coefficient of catchability” or “q” in the international notation, which is the proportionality constant which relates Catch-Per-Unit Effort (CPUE) to true stock density and also fishing effort to fishing mortality. Variations in catchability generally stem from (i) changes in the effectiveness of the effort and (ii) changes in the way the species interacts with the unit of effort, or the vulnerability of that type of fish to the fishing gear (Francis et al., 2003; Penn, 1984). Therefore, the catchability coefficient is a crucial parameter for the purpose of achieving more accurate stock assessment and fishery management outcomes (Arreguin-Sanchez, 1996; Zhou et al., 2008).
Penaeid shrimps support an important commercial trawl fishery of about 5 – 8.000 metric tonnes a year in Sofala Bank region, off central Mozambique (Fig. 1). The main species taken are Penaeus indicus and
Metapenaeus monoceros, which are fished mainly during the daylight hours, while four other species, Penaeus monodon, Penaeus semisulcatus, Penaeus latisulcatus and Penaeus japonicus, make up the
remainder of the catch. The latter three species are more active at night (Palha de Sousa et al., 2006), and tend to have a deeper spatial distribution, compared with the more daylight active species. For these reasons the fishery operates on a 24 - hr basis, and has historically operated over a fishing season of six to nine months each year starting in March each year. The closed season over the summer months protects the main stock of juvenile P. indicus shrimps as they migrate from the nursery areas and “recruit to the fishery”.
Unlike the daylight species that recruit to the fishery in the summer (November-March) the nocturnal species P. latisulcatus, P. japonicus and P. semisulcatus recruit to the fishery mostly during the period June-October each year. These species are known to have marked diel burrowing and emergence behaviors (Egusa and Yamamoto, 1961; Moller and Jones, 1975; Tanner and Deakin, 2001) controlled by light intensity, which influences their catchability. Thus, it is likely that catch rates from fishery monitoring programs and commercial trawling at different times of the day, may be influenced not only by variations in the abundance of these shrimps, but also by their behavior which influences the efficiency of trawling and therefore each species catchability. An understanding of the factors influencing catchability is therefore regarded as essential for relating stock abundance to observed CPUE (Ye and Mohammed, 1999), which can in turn provide a basis for more reliable stock assessments for fisheries management.
Among the few studies on catchability of penaeids are those carried out for P. latisulcatus in Australia (Penn, 1976) and for Penaeus semisulcatus in Kuwait (Bishop et al., 2008; Ye and Mohammed, 1999). In the Sofala bank shrimp fishery, no direct assessment of trawl efficiency has been carried out, and therefore information on this key aspect of catchability is not yet available.
For this study we carried out Leslie-DeLury depletion experiments similar to those undertaken by Joll and Penn (1990), Burridge et al., (2003), and Wakefield et al., (2007) in an equivalent offshore marine trawl fishery environment. This research was aimed at determining the efficiency of the typical industrial vessel otter trawl for the capture of each of the nocturnal shrimp species (P. japonicus and P. latisulcatus) taken in the Sofala Bank fishery, as the same or similar nocturnal species had been successfully examined using this technique. Similar but separate depletion experiments on the daylight active species (P. indicus, P.
monodon and M. monoceros) were also carried during daylight hours, but did not show significant
depletion over four days of fishing (Brito, 2010). This result for P. indicus and P. monodon was not unexpected as these species are reported to have limited burrowing capacity (Penn 1984) and are caught in daylight as well as at night. This lack of burrowing can be expected to allow these species be readily displaced by tidal flows, and therefore were unlikely to remain within the marked grid within the
experimental period. The M. monoceros data did show some depletion (approximate trawl efficiency of 18%), but the results were not significant, probably also due to the potential for tidal displacement as the species is typically caught with P. indicus in both daylight and at night.
The information produced for the nocturnal species was specifically designed to allow more accurate estimates of shrimp biomass from pre-recruitment surveys and therefore contribute to more effective management of these species which contribute to the nationally important Sofala Bank shrimp fishery (Palha de Sousa et al, 2006).
2. Materials and Methods
2.1. The study area
This study was carried out in coastal waters off central Mozambique (between latitudes 16o 21‟ S and 21o 00‟ S), in the Sofala Bank area, which supports the country‟s main ground for shrimp fishing (Fig. 1). The characteristics of the coastal environment and ecosystem as well as the fishery have been described in detail by Palha de Sousa et al., (2006); Lutjeharms, (2006); Leal et al., (2009).
2.2. Depletion Experiment
The depletion experiment was planned to occur over four consecutive nights in January 2009, to ensure the trawl coverage would sufficiently deplete of the stock present in the area. Unfortunately, weather restrictions forced the cancellation of the fourth night, which limited the trawl impact to three passes over the experimental area on consecutive nights. The January timing and offshore location was selected to coincide with the period of minimum recruitment for the nocturnal species in the Sofala Bank shrimp fishery. No recruitment into the experimental population is one of the key assumptions needing to be met for the Leslie-DeLury method (Leslie and Davis, 1939) to operate successfully.
To allow the results to be applied generally to industrial shrimp trawl catch data, the otter trawl nets used were the same as those of the major fishing company operating in the fishery. The chartered vessel “Pescamar XI”, used to undertake the experimental trawling was a typical Sofala Bank industrial trawler of 27.7 m in length and 185.6 Gross Registered Tonnes.
A rectangular experimental area of 312 m wide x 1850 m long was selected based on the criteria of having high abundance of shrimp of the target species. The area was located at a distance of about 24 km offshore (18o 02‟ S and 37 o 18‟E) with a depth of 32 m (Fig. 1) and oriented such that trawl directions were perpendicular to the coastline, with each pass (at each lane) being of approximately 20 minutes duration (Fig. 2). Trial trawls near the proposed area were undertaken to assess general catch rates in the area before selecting the experimental grid location. The grid area was then defined and marked out using a „differential global positioning system‟ (DGPS), which was also used to precisely control the vessel path throughout the experiment. The DGPS used had an indicated accuracy of plus or minus 4 m. The set of otter trawls used by the research vessel were of a typical Sofala Bank fishery design and had a combined opening width of 52 m, in a standard quad net configuration. The estimated distance between inner and outer boards was 26 m (13 m times two nets) with each pair of nets being towed from the booms on either side of the vessel. The distance (gap) between the inner otter boards of each pair of nets was estimated to be 1.5 m, and was not subject to trawling. After the six trawl passes over the selected area each night, the accumulated area not trawled was about 9 m (1.5 m times six passes) multiplied by the trawl distance of 1.850 m, or 3% of the total area (Fig. 2). As this theoretically un-fished area was relatively small and the path followed by the vessel would not be perfectly accurate, the un-fished area should not have negatively affected the results. However, this factor could result in a slight underestimate (about 3%) of the trawl efficiency if local dispersal resulted in shrimp moving into the small un-fished areas. The timetable for the six trawl hauls (vessel passes) on each of the three nights is set out in Table 1.
Figure 1. Location map of Sofala Bank fishery in Mozambique, east Africa, with the experimental site (solid triangle) near Macuze
marked.
Table 1. Daily program of trawls repeated for each of the three consecutive nights during the Leslie-DeLury experiment on Sofala
Bank. Start time Finish time Time Grouping 18:00 18:20 Dusk 20:00 20:20 Night 22:00 22:20 Night 0:00 0:20 Night 2:00 2:20 Night 4:00 4:20 Dawn
2.3. Catch sampling and data recorded
All penaeid shrimps caught during each trawl haul were sorted from the remainder of the catch and then separated by species. All specimens of each species were counted and retained on the vessel. Data applicable to the effort applied i.e. the time each trawl haul started and finished, vessel speed (over the ground), the wind speed and the average depth of each trawl was recorded from the instruments onboard the vessel. The tide heights for each trawl were recorded from the onboard computer displaying predicted
values for the nearest port of Macuse (17o 46‟ S, 37o 12 E), located 35 km north of the experimental site. The slope of the tidal harmonic model was used as an index of tidal speed as described by Dronkers (1964).
a)
26m Grid Northb)
1 8 5 0 m 1 1 2 2 3 3 4 4 5 5 6 6 321 m 52 m +1.5 mNumbered vessel pass lanes (1-6)
Figure 2. Schematic representation of the vessel Pescamar XI with (a) gear dimensions during fishing operations, and (b) the
experimental area (not to scale) with the vessel passes numbered. The arrows in each lane represent the direction of the trawl net (adapted from Wakefield et al., 2007).
2.4. Data processing and statistical analysis
Since fishing effort was consistent each night, the trawl efficiency was calculated as the slope of a Leslie plot (Leslie and Davis, 1939) with total numbers of shrimp specimens caught per night plotted against cumulative catch in numbers prior to that night. The 95 % confidence levels of the slopes (trawl efficiency) were calculated according to Snedecor and Cochran (1980).
The percentage of shrimp removed was then estimated by dividing the total numbers of shrimps available (value of intercept in x-axis) by the cumulative catch at the end of trawl n, (n = 18 at the end of the experiment).
A general linear model (GLM) of the form:
log (number/trawl) = ß0 + ß1x1 + ß2x2 + ß3x3 + ... + Ɛ ……….. (equation 1)
was used to assess the effects of the 10 independent variables available. These physical and environmental factors included the direction of tide: trawl against the tide (x1), trawl at slack tide (x2), trawl with the tide (x3); the time of trawling: dusk (x4), night (x5), dawn (x6); depth of trawling (x7); the wind speed (x8); tidal velocity(x9) and cumulative catch (x10). The ß are regression coefficients to be estimated.
3. Results
3.1. Depletion rates and trawl-efficiency
A total of three of the nocturnal penaeid shrimp species regularly taken in the fishery were found in the repeat-trawl experimental grid. P. japonicus, and P. latisulcatus appeared in moderate numbers, however only three specimens of P. semisulcatus were caught, and were therefore insufficient for further analysis. The impact of trawling on stocks in the experimental grid was evaluated by assessing the decline in the population during the course of the three nights of the experiment. Both P. latisulcatus and P. japonicus had more than 50 % of their population removed from the experimental grid after three nights of repeated trawling (Table 2). Due to the very small numbers of shrimp of both species caught in the dusk trawl (at 1800 hrs) these results effectively only relate to trawls two to six each night, and effectively reduced the experimental grid area to 267.5m x 1850m.
To decrease the level of variability in the data and simplify the interpretation of the results, the Leslie plots for estimation of trawl efficiency were done using each night‟s aggregate catch (and cumulative catch) rather than using the catches from individual trawls within each night. The linear relationship between number/trawl-night and previous nights cumulative catch, fitted reasonably well to the linear model (Leslie-DeLury method) for P. latisulcatus (R2 = 0.86) and P. japonicus (R2 = 0.76) (Fig. 3). These results suggest that trawl capture efficiencies for the two species were 24% and 23% respectively (Table 2) and that the more reliable estimate, based on very low 95% confidence levels for the Leslie-DeLury depletion model, was for P. japonicus.
Table 2. Estimated trawl efficiencies for P. latisulcatus and P. japonicus, the 95% confidence levels (C.L.) for the estimates and the proportion
of the population estimated to have been removed at the end of the experiment.
Species
Trawl
efficiency C.L.95% of Trawl efficiency
Population removed %
P. latisulcatus 0.2446 (24%) -0.9946, +1.4838 57
Figure 3. Leslie-DeLury plots from repeated-trawl (depletion experiment) for the nocturnally active shrimp species (a) P. latisulcatus
and (b) P. japonicus in the experimental area at Sofala Bank.
The six individual trawl catches obtained for each of the species during each night throughout the three nights of the experiment are shown in Fig. 4. As noted above, effectively no shrimp of either species were caught during the first trawl at dusk (18:00 hrs) each night, probably as a result of these nocturnal shrimp species not yet having emerged from the sediment to become available for capture by the trawl gear. For P.
latisulcatus the first significant catch was taken at 20:00 hrs each night however the numbers caught were
then variable through the night probably due to the small numbers involved. There was a clearer pattern in the catches of P. japonicus with higher catches at 20:00 hrs followed by a general decline and the lowest numbers occurring in the trawl at 04:00 hrs each night (Fig. 4).
Melicertus latisulcatus
y = -0.2446x + 108.29
R
2= 0.8629
0
50
100
150
0
50
100
150
200
Cumulative catch (numbers)
n
u
m
b
e
rs
/n
ig
h
t
Marsupenaeus japonicus
y = -0.2269x + 526.34
R
2= 0.7644
0
100
200
300
400
500
600
0
200
400
600
800
1000
Cumulative catch (numbers)
n
u
m
b
e
rs
/n
ig
h
t
Penaeus japonicus
Penaeus latisulcatus
0 20 40 60 80 100 120 140 160 180 200 18 20 22 24 2 4 18 20 22 24 2 4 18 20 22 24 2 4
Night 1 Night 2 Night 3
N u m b e r/ tr a w l Time (hours) M. latisulcatus M. japonicus
Figure 4. The catch rates of P. latisulcatus and P. japonicus observed over the three consecutive nights of the depletion experiment
carried out in the experimental area at Sofala Bank.
3.2. Effect of environmental factors on trawl efficiency
The effect of different environmental factors and the impact of the fishing gear (shown as cumulative catch) on the populations of shrimps in the experimental grid were investigated by a GLM for each species (Table 3). This GLM analysis showed that the effect of fishing gear only significantly explained the depletion of P. japonicus (t = -0.0002; p = 0.0041) and that trawling at dawn for this species was negative to the depletion rate, that is, catch rates of this species decreased at that period (t = -0.3027; p = 0.0041).
Table 3. Summary of results for the generalized linear model (GLM) testing for the effect of a number of environmental factors on
abundance (log number per trawl) at Sofala Bank. Note: only the significant parameters and their level of significance are shown.
Species Parameter Estimate SD
t-
student Prob(>|t|) F R2 Prob (>F)
P. latisulcatus intercept 1.2640 0.1139 11.0947 0.0000 1.5012 0.1200 0.2460 Cumulative catch -0.0011 0.0009 -1.2253 0.2461 P. japonicus intercept 2.0607 0.0565 36.4740 0.0000 10.81 0.6431 0.0020 trawling at dawn -0.3027 0.0858 -3.5303 0.0041 cumulative catch -0.0002 0.0000 -3.0268 0.0105 P.latisulcatus P.japonicus
4. Discussion
The application of Leslie-DeLury assessment method to the catches of both P. latisulcatus and P. japonicus from the offshore experimental area showed that a substantial depletion was achieved over the three nights, although this decline was not statistically significant for P. latisulcatus. This lack of significance for P.
latisulcatus appears to be due to the small numbers initially present and variability between the trawl
catches during each night, possibly due to the species occurring in small aggregations or patches (Gribble et al., 2007). The fourth night of fishing planned but abandoned due to weather constraints would possibly have improved the reliability of the depletion.
In addition to the catches declining generally over the three nights of the experiment, there were marked changes in catch rates within each night. For both species the catches were essentially negligible during the dusk trawl (18:00 hrs) in keeping with observations from the industrial fishery (Palha de Sousa et al., 2006), with the first consistent catches occurring at 20:00 hrs each night. While this lack of catch in the first trawl at 18:00 hrs each night reduced the effective area of the planned experimental grid, it was not considered likely to alter the basic reliability of the experiment. That is, the experimental design was effectively unchanged with the same area being trawled on each of the three nights.
The within night variation was most evident for P. japonicus, where catches showed a very distinct peak at 20:00 hrs during nights one and two. Catches between 20:00 hrs and 02:00 hrs each night were also generally higher than the pre-dawn trawl (at 04:00 hrs), which was consistently the lowest during the night. Since fishing effort was constant during the depletion experiment, these changes in catch-rate within each night are most likely the result of changes in the species nocturnal activity patterns, which have affected their vulnerability to capture by the trawl. The alternative hypotheses to explain this effect are that the shrimp have either burrowed at varying rates into the substrate or dispersed higher into the water column and therefore moved out of the path of the net. Burrowing in soft sand during periods of high light intensity, followed by emergence in darkness is a behavior common for penaeid species at least in aquaria experiments (Egusa and Yamamoto, 1961; Wassenberg and Hill, 1994; Primavera and Lebata 2000; Aguzzi et al., 2007) and supports the burrowing hypothesis. Further, Penn (1984) has reported that P.
latisulcatus is nocturnal and remains fully buried during the day and that species very similar to P. japonicus are also capable of burrowing into the substrate during daylight, sufficiently to reduce their
vulnerability to trawl capture. In the Sofala Bank fishery industrial catch rates for these species are negligible during the daylight when other more pelagic “banana” or white shrimp species are taken (Palha de Sousa et al., 2006) also suggesting that they remain buried in the substrate during the day rather than moving up into the water column. On this basis, we suggest that the changing light conditions from dusk (18:00 hrs) to complete dark/night (20:00 hrs) at Sofala Bank has triggered the emergence of previously buried shrimps, and allowed them to become vulnerable to the trawl gear being used.
Before accepting the rates of depletion and trawl efficiencies suggested by this experiment, the potential for bias from any breakdown in the assumptions inherent in the depletion methodology need to be considered. For this method, these assumptions (Leslie and Davis, 1939) were that the experimental population is closed during the experimental period i.e. there is no migration of shrimp into or out of the area. Secondly, the method assumes that there will be no significant losses from the experimental population from natural mortality during the course of the experiment. Because of the short time frame of three days, these mortality losses are unlikely to be significant, however local migration into or out of the experimental area cannot be ruled out. In terms of migration/emigration, shrimp movements into the experimental area during the course of the trial are more likely, as the depletion process reduces the local abundance in the experimental area. As a consequence, any local movements from the surrounding areas (at the original abundance) are more likely to artificially increase the population in the grid during the course of the experimental depletion. If this has occurred, the resulting estimates of trawl efficiency are likely to been slightly underestimated and the experimental population size over-estimated.
Based on the above assessment and possible sources of bias, it is suggested that the trawl efficiency for P.
japonicus and P. latisulcatus in the Sofala bank fishery is likely to be a minimum of about 25%. These
trawl efficiencies of 34% and 50% in two depletion experiments for one of the Sofala Bank species (P.
latisulcatus). These higher efficiencies (Penn, pers. com.) are to be expected as trawls being used in that
experiment were specifically designed to capture fully demersal shrimp species, P. latisulcatus and
Penaeus esculentus (a species with similar behavioral to P. japonicus, Penn 1984), rather than the trawls
used in the current experiment which were designed for the more pelagic white shrimp species taken in the Sofala bank fishery.
It is well recognized that in regular fishing operations, catchability can vary for reasons associated with abundance, species behavior, population biology including its dynamics, even quality and amount of fishing effort, fishing strategy and environmental conditions (Arreguin-Sanchez, 1996). This study has been the first attempt to quantify the important effects of shrimp behavior on trawl efficiency and catchability in the Sofala Bank fishery and has provided useful data for the two dominant nocturnal shrimp species taken. The results confirm that the efficiency of the trawl gear when used to capture the nocturnal species is well below the 100% level often assumed when using of trawl catch per unit of effort as a measure of stock abundance. For the two major nocturnal species (P. japonicus and P. latisulcatus) assessed, the trawl net designs currently in use in industrial fishery and for research surveys appear likely to be capturing in the order of 25 % of the shrimp in the path of the net on average over a night. This level of efficiency is considered to reflect the burrowing behavior of these nocturnally active species, which prevents their capture during daylight, but also appears to influence/vary their vulnerability to the fishing gear throughout the night.
Specifically, the information on the trawl efficiency aspect of catchability from this study has the potential to significantly improve the biomass estimates for P. japonicus and P. latisulcatus obtained from the annual pre-season recruitment surveys (in January/February) of the Sofala Bank fishing grounds. That is, the analysis of these survey data sets, currently involves the use of the “swept area method” to assess stock size, which in the absence of specific catchability information, has involved the implicit assumption that all shrimp in the path of the trawl net are caught, i.e. implying that the CPUE is a direct reflection of abundance. The more realistic measures of trawl efficiency now available, will not only enable the survey based biomass estimates for the nocturnal species to be recalibrated, but will also have the potential to significantly improve the reliability of CPUE based stock assessments of Sofala Bank fishery and other similar shrimp fisheries generally.
5.Acknowledgments
Captain of „Pescamar XI‟, Mr. Laureano Rial and his crew are thanked for their cooperation on board that made possible the experimental work. Mr. Dionisio Varela and other technical staff of the Mozambique Fisheries Research Institute (IIP, in Portuguese acronym) helped in sorting out samples during this labor-intensive and time demanding experiment. Lizette Palha de Sousa, Silvia Abdula, Sonia Nordez (all of IIP) made helpful comments at various stages of the project and provided important literature. Funding for this study was provided by the government of Mozambique through the Fisheries Research Institute.
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