Prawn Catch Research Articles

Importance of estuarine mangroves to juvenile banana prawns

Offshore catches of banana prawns, Penaeus merguiensis, are correlated with the extent of mangrove forests. However, recent evaluation has questioned whether the apparent relationship between juvenile penaeids and mangroves reflects specific utilisation of mangroves or just the use of shallow, organically rich, muddy habitats. We investigated this by focussing on juvenile P. merguiensis within 30 mangrove estuaries spanning 650 km of the coast of north-eastern Australia. We investigated a range of hierarchically clustered spatial scales and within-estuary spatial resolutions, as well as variables representing a variety of estuary structural factors, anthropogenic impacts, and particular hypotheses about the ways in which mangroves could influence P. merguiensis catch per unit effort (CPUE). Estuary to estuary differences, rather than climatic zone or the proximity of other estuaries, was the major large scale spatial influence on CPUE. At the among-estuaries scale mangrove extent appeared to influence CPUE but was extensively confounded with the effects of two non-mangrove variables; intertidal extent and substrate type. The fact that 3 alternative measures of connectivity with mangrove forests were not influential, points to the importance of the non-mangrove variables rather than mangrove extent. At the within-estuary scale, P. merguiensis CPUE was correlated with the extent of shallow water but not with mangrove variables. The spatial and temporal extent of sampling support a strong conclusion that factors associated with mangroves alone do not drive abundances of juvenile prawns. Nevertheless, despite being the dominant habitat, mangroves are only one of a mosaic of interacting habitats occurring in the tropical estuaries inhabited by juvenile penaeids (Sheaves, 2009), so causal relationships are complex and difficult to define unambiguously.

A trap with a twist: evaluating a bycatch reduction device to prevent rockfish capture in crustacean traps

Abstract Favaro, B., Duff, S. D., and Côté, I. M. 2013. A trap with a twist: evaluating a bycatch reduction device to prevent rockfish capture in crustacean traps. – ICES Journal of Marine Science, 70:114–122. Bycatch, or the incidental capture of non-target species, occurs in fisheries around the world, with often detrimental ecological consequences. Bycatch reduction devices (BRDs) that increase catch specificity have been used successfully in some fisheries, and the development of such devices remains an important component of the global effort to reduce bycatch rates. We tested novel devices designed to exclude juvenile rockfish (Sebastes spp.) from traps used to catch spot prawns (Pandalus platyceros), a commercially important species in British Columbia, Canada. The devices included reductions in trap opening sizes and novel bent-tunnel openings. Reducing trap opening size did not affect bycatch rates of rockfish or other non-target fish species. In contrast, bent-tunnel BRDs eliminated rockfish bycatch, and two of the bent-tunnel variants also excluded other fish species. However, prawn catch rates were reduced in all modified gear, and large prawns were often excluded more than small prawns. Videos recorded in situ revealed that prawn attempts to enter traps took longer and were more likely to fail in BRD-equipped than in unmodified traps. We conclude that bent-tunnel BRDs have the potential to be useful, but improvements are needed to increase prawn catch to levels similar to that of unmodified traps.

Characteristics and value of the Thukela Banks crustacean and linefish fisheries, and the potential impacts of changes in river flow

This study describes the Thukela Banks crustacean and linefish fisheries and investigates the potential impacts of reduced flow from the Thukela River on the value of these fisheries. Data were obtained from published and unpublished material, key informants and government records. The crustacean fishery employs about 300 people, and comprises an inshore and offshore fishery. Inshore catches and effort are strongly seasonal, peaking in April to June. The probability of fishing in any particular month is positively correlated with catch per unit effort. Average overall annual catch (inshore and offshore) was estimated to be in the region of 700 tonnes for 1992–2002 with a gross output of about R36.7 million per annum, and a gross value added (net output) of R13.8 million (1 US$ = R6.40; 2003). Inshore prawns were estimated to contribute R8.5 million of the gross output. The Thukela Banks was estimated to contribute R4.5 million and R35 million (rand value in 2003) to the gross output of the commercial linefishery and boat-based recreational angling fishery respectively. Freshwater reduction scenarios yielded 0.7–11% reductions in prawn catches, which translated into only a 1–2% drop in the annual value of the fishery. The latter was attributed to the fishery's diversity (predominantly the buffering effect of bycatch) and the fact that the stocks of the species targeted by the offshore component are largely independent of flow. Catches and value of the recreational boat-based linefishery were predicted to remain fairly constant irrespective of any changes in flow. The commercial linefishery was the most responsive, with a 20% decline in total catch and 17% value predicted for the most extreme flow reduction scenario.

Changes in the fish fauna of a tropical Australian estuary since 1990 with reference to prawn predators and environmental change

As part of a study of the reasons for declining commercial banana prawn catches in the Weipa area of the Gulf of Carpentaria, and in order to assess whether changes in predator numbers may be a factor influencing prawn numbers, the species composition, numbers and biomasses of fishes in the Embley estuary were re-examined in February 2005 for comparison with the results of a study between 1986 and 1990. The new study duplicated previous methods and used the same sampling sites and the same gear employed during February sampling periods from 1986 to 1990. Overall, the species diversity and catch rates, as well as the numbers of penaeid prawn predators had changed little over 20 years. The overall catch rate for 2005 was 37.4 g m −1 h −1, not significantly different from the overall catch rate of 32.8 g m −1 h −1 (SE 6.6) for the period from 1987 to 1989. The results are discussed in relation to environmental factors and climate change, as well as the substantial declines in prawn catches, which had fallen from over 800 tonnes per annum prior to 1998, to about 70 tonnes per annum.

Bycatch of rockfish and other species in British Columbia spot prawn traps: Preliminary assessment using research traps

Abstract The spot prawn (Pandalus platyceros) trap fishery in British Columbia is endorsed by conservation organizations owing to the assumption of minimal bycatch. However, reported capture of juvenile rockfish (Sebastes spp.) in prawn traps has raised concern due to declines in abundance of many rockfish stocks. We document the bycatch observed in a 10-year (1999–2008) fishery-independent research survey that employed traps that are similar to the traps used in the commercial spot prawn fishery. Research traps produced 0.16–0.20 kg of non-target catch per kg of spot prawn catch, with bycatch consisting mainly of a variety of molluscs, non-target crustaceans, echinoderms, and fish. The overall rate of rockfish catch was low—0.015 rockfish per trap. The annual rate of rockfish bycatch has increased since 2004, to 0.039 rockfish per trap in 2008, while catch rates of other species have remained relatively constant. Our results confirm that spot prawn traps produce a low amount of bycatch by weight. However, they also suggest that rockfish mortality due to prawn trapping should be quantified in the commercial prawn fishery to determine how this source of mortality may affect rockfish stocks. Furthermore, research into bycatch reduction technology to improve trap selectivity, and thus reduce rockfish bycatch, would be desirable.

Assessment of the impacts of a new artificial lake mouth on the hydrobiology and fisheries of Chilika Lake, India

AbstractThe degraded state of the fragile ecosystem of Chilika Lake, located on the east coast of India, was undergoing restoration through an effective hydrological intervention during the year 2000. Studies on the lake’s hydrobiology and fisheries for the period of 7 years before and 7 years after the hydrological intervention indicated a rapid recovery of the lake fishery immediately after opening of the new lake mouth, with a sixfold increase in the average annual fish landings. During 2000–2001 to 2006–2007, the average fisheries output (11 051.3 t), catch per unit effort (6.2 kg boat−1 day−1), the economic valuation of the average annual catch (637 million rupees) and productivity (11.97 t km−2) exhibited dramatic increases of 498%, 464%, 1177% and 498.5%, respectively, compared with the 7‐year pre‐intervention data. Multivariate statistical analysis inferred that most of the lake’s environmental variables are strongly associated with salinity factor, which seems to have governed the lake ecology. The salinity dynamics of the lake are governed by both freshwater inflows and seawater ingress through the new artificial lake mouth. Correlation analysis indicated that salinity was positively correlated with prawn catch (R2 = 0.542; d.f. = 25; P < 0.01), crab catch (R2 = 0.628; d.f. = 25; P < 0.001) and fish catch (R2 = 0.476; d.f. = 25, P < 0.05). The average increase in the salinity regime (43.8%) for the lake during the post‐hydrological intervention period, compared with the pre‐hydrological intervention period, appears to have positively impacted the fish, prawn and mud crab catches. A gradual decrease in total fisheries output since 2005–2006, however, was attributed mainly to a continuing increase in destructive fishing practices in the absence of any conservation and regulatory measures for fishing, and large‐scale collection of shrimp juveniles from the outer channel for shrimp aquaculture. Thus, carefully planned conservation and regulation measures must be ensured, with active participation of local communities during this early phase of lake restoration. In the absence of such measures, the present scenario of fisheries enhancement might not be sustained over the long term.

Modelling the possible effects of climate change on an Australian multi-fleet prawn fishery

The relationship between fisheries and climate has been given renewed emphasis owing to increasing concern regarding anthropogenically induced climate change. This relationship is particularly important for estuarine fisheries, where there are documented correlations between river discharge and productivity. The commercial catch of school prawns (Metapenaeus macleayi) has been shown to be positively correlated with the rates of river discharge in northern New South Wales, Australia. In the present study, a simulation model was developed to analyse the dynamics of the stock for 10 years under alternative river discharge scenarios, and the effectiveness of a series of management strategies under these scenarios was examined. A size-based metapopulation model was developed that incorporated the dynamics of school prawn populations in three habitats being harvested by three different fishing methods. The model indicated that both the growth and movement of prawns were affected by the rates of river discharge, and that higher rates of river discharge usually generated increased commercial catches, but this outcome was not certain. It was concluded that the population does not appear to be overexploited and that none of the three alternative management strategies performed better within the model than the current spatio-temporal closures, even under a wide range of river discharge scenarios.

The first record of the Jinga shrimp, Metapenaeus affinis (H. Milne Edwards, 1837) (Decapoda, Penaeidae) from the Mediterranean Sea

Of the nearly 600 alien marine species of Metazoa that have been recorded in the Mediterranean Sea, two-thirds have established durable populations of ever larger range (Galil, 2008a, b). The majority of the aliens originate from the Indo Pacific region and have entered the Mediterranean through the Suez Canal (Ery threan aliens). Nine species of alien penaeid prawns occur in the Levant Basin, eastern Mediterranean, all of them known from the Turkish coast: Marsupenaeus japonicus (Bate, 1888), Metapenaeus monoceros (Fabricius, 1798), Penaeus semi sulcatus De Haan, 1844, Melicertus hathor (Burkenroad, 1959), Metapenaeop sis aegyptia Galil, 1990, Metapenaeopsis mogiensis consobrina (Nobili, 1904), Metapenaeus stebbingi (Nobili, 1904), Trachysalambria palaestinensis (Steinitz, 1932), and Fenneropenaeus merguiensis (De Man, 1888) (cf. Galil, 2008a, b). All the alien penaeids but one (E merguiensis) have entered the Mediterranean through the Suez Canal. The first three species are highly valued commercially and consti tute most of the prawn catch off southeastern Turkey (Duruer et al., 2008). Extensive studies conducted in 2003-2005 of the commercial prawn trammel net fishery targeting the native Mediterranean prawn, Melicertus kerathurus (Forsk?l, 1775) in Izmir Bay, have revealed no alien prawns, either in the landed catch or in the discard (Gok?e & Metin, 2007; Akyol, 2008).

Spatio-temporal modelling of prawns in Albatross Bay, Karumba and Mornington Island

The seasonal life-history patterns of penaeid prawns are complex and there are distinct differences between species. Better knowledge and understanding of offshore migration to the fishing grounds of penaeid prawns is particularly important for the operation and management of the Northern Prawn Fishery, Australia. Knowledge of the location of prawns during the critical spawning period of the year is important, especially since recent work suggests that inshore areas are critical for effective spawning. This paper explores factors, such as rainfall, water depth and season, impacting prawn migratory patterns, and models the spatial and temporal distribution of three commercially valuable species of prawns ( Penaeus merguiensis, Penaeus semisulcatus and Penaeus esculentus) in Albatross Bay, Karumba and Mornington Island in the Northern Prawn Fishery using modern statistical methodology. The methodology allows for flexible functional relationships between variables, including univariate smooth terms and smooth terms for interactions between covariates: a major advancement on the methodology used in previous analyses of spatial distribution of prawns in the Gulf of Carpentaria. The models interpolate well, but expose the problems associated with extrapolation beyond the data. The results suggest that there are spawners in the important inshore waters during the critical spawning period of the year, although there is no strong evidence for major congregations of prawns in these important inshore regions compared to more offshore regions. Rainfall was significant in many of the models, particularly those describing the distribution of Penaeus merguiensis. Increased rainfall mostly had a positive effect on prawn catch rates, possibly by stimulating prawns to move out of shallow coastal areas into the fishing area. However, in some cases, high levels of rainfall had a negative effect which may be due to storms associated with the heavy rainfall decreasing the effectiveness of the fishing fleet. The analyses showed no signs of major inshore migrations associated with spawning, but there were certainly some spawners close inshore at the critical spawning period.

Evaluating ecosystem-based management options: Effects of trawling in Torres Strait, Australia

A suite of management options for a prawn trawl fishery in Torres Strait, Australia was assessed for impacts on the benthic fauna using a dynamic management strategy evaluation approach. The specification of the management options was gained through consultation with stakeholders. Data for the model was drawn from several sources: the fleet data from fishery logbooks and satellite vessel monitoring systems, benthic depletion rates from trawl-down experiments, benthic recovery rates from post-experiment recovery monitoring studies, and benthic distribution from large-scale benthic surveys. Although there were large uncertainties in the resulting indicators, robust measures relevant to management were obtained by taking ratios relative to the status quo. The management control with the biggest effect was total effort; reducing trawl effort always led to increases in benthic faunal density of up to 10%. Spatial closures had a smaller benefit of up to 2%. The effect of closing a set of buffer zones around reefs to trawling was indistinguishable from the status quo option. Closing a larger area, however, was largely beneficial especially for sea cucumbers. When the spatial distributions of fauna prior to fishing were accounted for, fauna with distributions positively correlated with effort improved relative to those negatively correlated. The reduction in prawn catch under effort reduction scenarios could be ameliorated by introducing temporal closures over the full-moon period.

Contraction of the banana prawn (Penaeus merguiensis) fishery of Albatross Bay in the Gulf of Carpentaria, Australia

When the biomass and area occupied by a stock decline together, catch rates can remain high (hyperstability) and management with effort controls may be ineffectual. Banana prawn (Penaeus merguiensis) catches declined from 2000 until 2005 in the Albatross Bay area in the Gulf of Carpentaria (GOC), Australia. Data from commercial logbooks were used to investigate historical changes in the banana prawn fishery in this and other regions of the Northern Prawn Fishery to infer the potential causes of this decline. Data since 1970 were analysed using: (1) the mapping of catch and effort; and (2) normalised rank order catch curves, to determine the distribution of catches across fishing areas. These analyses show that there has been a marked contraction of the Albatross Bay fishery over 33 years of fishing into the centre of a stable ‘hotspot’, suggesting a potential mechanism for the observed negative relationship between catchability and biomass. We believe this is the first observation of a Peneaus prawn fishing ground contracting as biomass declines, supporting the view that the contraction of an area occupied by a stock as biomass declines, is a generalised phenomena observed widely across fisheries resources and not a dynamic confined to certain finfish and molluscs.

Can bycatch reduction devices be implemented successfully on prawn trawlers in the Western Indian Ocean?

Bycatch reduction devices (BRDs) are increasingly being used in prawn trawl fisheries worldwide. This paper describes an experiment with a Nordmøre grid and a square-mesh panel on a prawn trawler off Moçambique. Although numbers of hauls that caught elasmobranchs were low, 75% of hauls with grids caught fewer large rays than those without grids; hauls using grids caught no large sharks at all. In its final configuration, the grid respectively reduced prawn, discard and retained fish catches by 5%, 25% and 32% relative to control hauls; the square-mesh panel increased prawn catches by 3% and respectively reduced discard and retained fish catches by 23% and 18% relative to control hauls. Changes in prawn catches were not statistically significant; however, when the grid and the square-mesh panel were used together, they reduced prawn, discard and retained fish catches by 25%, 47% and 26% respectively relative to control hauls (all significant except for retained fish catches). Overall, the Nordmøre grid successfully released large elasmobranchs, and the grid and the square-mesh panel individually reduced bycatch without reducing prawn catches. In combination, the two BRDs also reduced bycatch, but at the cost of substantially reduced prawn catches. The results indicate that there is scope for the wider use of BRDs on prawn trawlers in the Western Indian Ocean region, although this would require increased support from management agencies and industry.

Quantifying the effects of bycatch reduction devices in Queensland's (Australia) shallow water eastern king prawn ( Penaeus plebejus) trawl fishery

This study presents results from an experimental 10-day research charter that was designed to quantify the effects of (a) a turtle excluder device (TED), (b) a radial escape section bycatch reduction device (BRD) and (c) both devices together, on bycatch and prawn catch rates in the Queensland shallow water eastern king prawn ( Penaeus plebejus) trawl fishery. The bycatch was comprised of 250 taxa, mainly gurnards, whiting, lizard fish, flathead, dragonets, portunid crabs, turretfish and flounders. The observed mean catch rates of bycatch and marketable eastern king prawns from the standard trawl net (i.e., net with no TED or BRD) used during the charter were 11.06 kg/hectare (ha −1) (S.E. 0.90) swept by the trawl gear and 0.94 kg ha −1, respectively. For the range of depths sampled (20.1–90.7 m), bycatch rates declined significantly at a rate of 0.14 kg ha −1 for every 1 m increase in depth, while prawn catch rates were unaffected. When both the TED and radial escape section BRD were used together, the bycatch rate declined by 24% compared to a standard net, but at a 20% reduction in marketable prawn catch rate. The largest reductions were achieved for stout whiting Sillago robusta (57% reduction) and yellowtail scad Trachurus novaezelandiae (32% reduction). Multidimensional scaling and analysis of similarities revealed that bycatch assemblages differed significantly between depths and latitude, but not between the different combinations of bycatch reduction devices. Despite the lowered prawn catch rates, the reduced bycatch rates are promising, particularly for S. robusta, which is targeted in another fishery. Prawn trawl operators are not permitted to retain S. robusta and the devices examined herein offer the potential to significantly reduce the incidental fishing mortality that this species experiences.

A review of kuruma prawn Penaeus japonicus stock enhancement in Japan

We reviewed the stock enhancement programme for kuruma prawns Penaeus japonicus in Japan. The annual catches of kuruma prawns for the whole of Japan have fluctuated between ∼1300 and 3700 t since the mid-1960s. The kuruma prawn stock enhancement programme started at the Seto Inland Sea in western Japan in 1964. Since then, annual releases of juveniles increased to ∼300 million p.a. and then decreased to 140 million p.a. There has been a corresponding increase in size at release. Analysis of catch and release histories for the whole of Japan suggested that population sizes of kuruma prawns fluctuated depending on the condition of the coastal marine environment, particularly the tidal nursery habitats, and that hatchery releases have not succeeded in augmenting total production of this species. We examined 40 case studies from recent release experiments that used 2 methods to mark kuruma prawns: coded wire tags, and uropod clipping. Estimates of recapture rates for released prawns ranged from 0.0 to 22.1%, but exceeded 10% in only two cases. Higher recapture rates were obtained at a smaller release size in regions where there were more extensive tidal nursery grounds. Analysis of economic efficiency showed that stock enhancement of kuruma prawn was only profitable in 5 of 40 cases. The results of this synthesis highlight the importance of the quality and quantity of release habitats and the need for restoration of nursery habitats. They also show marked variation in the success of releases and emphasise the need for research to develop appropriate strategies to improve survival of cultured juveniles in those places where releases are predicted to provide benefits.

An overview of commercial fishing in Lake Illawarra: trends in reported catch and effort, bycatch and discarding

NSW estuaries have supported valuable commercial fisheries since the early 1800’s. The NSW estuarine commercial fishery is the most complex multi-method and multi-species fishery in NSW, producing in 2001/02 approximately 3900 t of finfish and 890 t of invertebrates, valued at over $18 million. The commercial fishery in Lake Illawarra is relatively productive and important to the total estuarine commercial catch in NSW. Between 1997/98 and 2001/02, the average reported commercial catch of finfish, prawns and blue swimmer crab was 120 t, 17 t, 12 t, respectively, which was 2.9%, 2.0% and 6.5% respectively, of the total estuarine catch in NSW. The finfish catch was dominated by sea mullet (47%) and luderick (10%), which is typical of the NSW estuarine fishery in general, and dusky flathead, silver biddy, bream and river garfish also contributed > 5 t each per annum. Most finfish were caught in gillnets (75% of catch), whereas most prawns were caught in running nets (76% of catch). Bycatch and discarding in gillnets and prawning gears in Lake Illawarra was similar to other estuaries in NSW. Current research to improve the selectivities of prawn gears and changes in regulations concerning gillnets should reduce bycatch and discarding in some gear types.

Bioeconomic modelling and risk assessment of tiger prawn (Penaeus esculentus) stock enhancement in Exmouth Gulf, Australia

A bioeconomic model was developed to evaluate the potential performance of brown tiger prawn stock enhancement in Exmouth Gulf, Australia. This paper presents the framework for the bioeconomic model and risk assessment for all components of a stock enhancement operation, i.e. hatchery, grow-out, releasing, population dynamics, fishery, and monitoring, for a commercial scale enhancement of about 100 metric tonnes, a 25% increase in average annual catch in Exmouth Gulf. The model incorporates uncertainty in estimates of parameters by using a distribution for the parameter over a certain range, based on experiments, published data, or similar studies. Monte Carlo simulation was then used to quantify the effects of these uncertainties on the model-output and on the economic potential of a particular production target. The model incorporates density-dependent effects in the nursery grounds of brown tiger prawns. The results predict that a release of 21 million 1 g prawns would produce an estimated enhanced prawn catch of about 100 t. This scale of enhancement has a 66.5% chance of making a profit. The largest contributor to the overall uncertainty of the enhanced prawn catch was the post-release mortality, followed by the density-dependent mortality caused by released prawns. These two mortality rates are most difficult to estimate in practice and are much under-researched in stock enhancement.

Selectivity of penaeid trap nets used in south eastern Australia

Two experiments were done to estimate the selectivity of commercial and modified trap-net configurations in New South Wales (NSW), southeastern Australia. In the first experiment, a commercial trap net made entirely from 25 mm mesh and designed for use in shallow water was alternatively fished with a fine-meshed (9.5 mm netting) trap net (used as a control). In the second experiment, two trap-net configurations designed for use in deeper water and comprising the same anterior section (made from 25 mm mesh), but with different bunts made from (i) the conventional 25 mm mesh and (ii) 31 mm mesh were alternately fished against the control. Both of the conventional trap nets (comprising 25 mm mesh throughout) had low amounts of bycatch and similarly selected eastern king Penaeus plebejus, greasyback Metapenaeus bennettae and school prawns Metapenaeus macleayi across narrow selection ranges (< 3.4 mm) and at 50% retention lengths (between 18.53 and 21.50 mm) that were larger than the average commercially-accepted sizes (15-17 mm CL). Analyses of the selectivities and relative efficiencies of the trap-net configurations comprising the 25 and 31 mm bunts showed no benefit, in terms of maintaining prawn catches and reducing unwanted bycatch, associated with increasing mesh size in these gears. The utility of trap nets for selectively harvesting penaeids is discussed. We conclude that this type of fishing gear appears to have few deleterious impacts.

Relationship between mangrove abundance and tropical prawn production: a re-evaluation

Despite decades of research, the relationship between mangrove abundance and fishery productivity in nearshore environments is still controversial. A new analysis was made to explore the relationship between prawn catch in 1985-2001 in 37 countries and mangrove abundance, using the principal components-regression approach. Principal components reduced mangrove abundance and other predictor variables (latitude, tidal amplitude, coastline length, population size, rainfall, and temperature) to four orthogonal principal components, accounting for 78% of the overall data variation. Representation of mangrove abundance in both absolute (total area of mangroves) and relative (mangrove area standardized against length of coastline) terms also helps elucidate the effect of mangrove abundance on prawn catch. Regression analysis with prawn catch using these four principal components suggests that the extent of intertidal areas and organic matter availability as represented by tidal amplitude rather than relative mangrove abundance per se have a stronger influence on prawn catch in tropical nearshore environments. This approach avoids some of the common problems of past analyses, such as multi-collinearity of the predictor variables.

Use of generalized linear models in analyzing the catch and effort data on the western king prawn Penaeus latisulcatus Kishinouye in the Gulf St. Vincent, Australia

Abstract The western king prawn Penaeus latisulcatus occurs in coastal waters off Western Australia, the Northern Territory, Queensland, New South Wales, and South Australia. In South Australia, it supports a commercial trawl fishery with an annual landed value of about AU$ 35 million. In this paper, I derive generalized linear models from an observational model of a sex-, age- and time-dependent model for a single prawn population, provide them with a firm theoretical basis, and make explicit some of their assumptions. Following an exploratory analysis by use of a generalized additive model, I use generalized linear models to analyze the commercial catch and effort data for the changes in its stock size with time, in fishing efficiency among vessels, in catch with time of day, fishing effort and area, and the effects of searching on catch. The results obtained by assuming that the catch follows independent log-gamma, log-normal, log-negative binomial, or log-Poisson distribution were similar. The relative exploitable biomass in the population immediately before the fishery was closed in 1991 was 31% of that when it was resumed in December 1993, remained at about the same level of December 1993 until 30 June 1995, slightly declined from 1 July 1995 to 30 June 1997, and was 81% of the level of December 1993 from 1 January 1998 onward. The relative instantaneous rate of fishing mortality of prawns of a particular sex and age was largely unchanged, but became less variable with time. Fishing efficiency was similar among all vessels. Normal fishing shots (using three nets) produced about three times the catch of searching shots (using only one net). Fishers learned during a sequence of shots, for prawn catch initially increased with shot number and then leveled off. A significant variation in catch with the time of day was detected, with the maximum catch occurring at 22 h. Finally, catch did not change proportionally with fishing effort; a 1% of change in effort led only to a nearly 0.9% of change in catch, so that raw catch per unit of effort decreased with fishing effort and is a biased index of the size of a prawn population. This work suggests that the current level of fishing effort cannot sustain the exploitable biomass of the population at the level of December 1993.

Use of generalized linear models in analyzing the catch and effort data on the western king prawn Penaeus latisulcatus Kishinouye in the Gulf St. Vincent, Australia

The western king prawn Penaeus latisulcatus occurs in coastal waters off Western Australia, the Northern Territory, Queensland, New South Wales, and South Australia. In South Australia, it supports a commercial trawl fishery with an annual landed value of about AU$ 35 million. In this paper, I derive generalized linear models from an observational model of a sex-, age- and time-dependent model for a single prawn population, provide them with a firm theoretical basis, and make explicit some of their assumptions. Following an exploratory analysis by use of a generalized additive model, I use generalized linear models to analyze the commercial catch and effort data for the changes in its stock size with time, in fishing efficiency among vessels, in catch with time of day, fishing effort and area, and the effects of searching on catch. The results obtained by assuming that the catch follows independent log-gamma, log-normal, log-negative binomial, or log-Poisson distribution were similar. The relative exploitable biomass in the population immediately before the fishery was closed in 1991 was 31% of that when it was resumed in December 1993, remained at about the same level of December 1993 until 30 June 1995, slightly declined from 1 July 1995 to 30 June 1997, and was 81% of the level of December 1993 from 1 January 1998 onward. The relative instantaneous rate of fishing mortality of prawns of a particular sex and age was largely unchanged, but became less variable with time. Fishing efficiency was similar among all vessels. Normal fishing shots (using three nets) produced about three times the catch of searching shots (using only one net). Fishers learned during a sequence of shots, for prawn catch initially increased with shot number and then leveled off. A significant variation in catch with the time of day was detected, with the maximum catch occurring at 22 h. Finally, catch did not change proportionally with fishing effort; a 1% of change in effort led only to a nearly 0.9% of change in catch, so that raw catch per unit of effort decreased with fishing effort and is a biased index of the size of a prawn population. This work suggests that the current level of fishing effort cannot sustain the exploitable biomass of the population at the level of December 1993.