The balancing act of captive breeding programmes: salmon stocking and angler catch statistics

The debate over Atlantic salmon, Salmo salar L., stocking in Britain centres on the trade‐off between enhancing rod fisheries and harming wild populations. This article informs the debate by quantifying the relationship between stocking and angler catch statistics for 62 rivers over 15 years. After controlling for environmental factors affecting adult abundance, the 42 rivers with stocking had non‐significantly lower mean catch statistics than the 20 rivers without stocking. This difference increased with the age of stocked fish. Among stocked rivers, weak relationships between mean stocking effort and catch statistics also became more negative with the age of stocked fish. For stocked rivers, there was no evidence for a generally positive relationship between annual stocking efforts and catch statistics. Those rivers for which stocking appeared to improve annual rod catches tended to have lower than expected mean rod catches. The results suggest the damage inflicted on wild salmon populations by stocking is not balanced by detectable benefits to rod fisheries.     

The effects of the net fishery closure on angling catch in the River Hvítá, Iceland

Between 1991 and 2000, angling associations on the tributaries of the River Hvítá leased net fishery rights in the Hvítá mainstem, with the aim of eliminating net fishery harvest and improving the rod catch. The rod catch and net catch in the Hvítá system were significantly correlated (r = 0.94; P < 0.001) over the 10‐year period prior to (1981–1990) closure of the net fishery. The rod fishery in the tributaries of the River Hvítá was also significantly correlated to the rod fishery in selected groups of rivers in west (r = 0.80; P < 0.01) and north (r = 0.73; P < 0.05) before the closure. Significant increases (P < 0.01) were observed in rod catches in the Hvítá tributaries between 1991 and 2000 after the closure, while rod catches in control regions decreased. Based on evaluation of rod catch trends before and after the closure, it was estimated that the net fishery lease increased rod catches in the tributaries between 1773 and 2175 fish (28–35%). The increase in rod catches also suggested that the rod fishery may be taking 39–52% of the estimated previous net catch. The high price paid annually for the net fishery lease just to eliminate net fishing (€135 000) reflects the high value of rod caught salmon compared with salmon caught by the net fishery.     

A preliminary appraisal of the contribution of recreational fishing to the fisheries sector in north-west Trinidad

Abstract An assessment of the recreational fishery in north-west Trinidad was made by questioning participants about the principal fishing methods used, location of activities, catches, effort and cost of fishing. Surface handline methods (live baiting [à la vive] and trolling) and rod and line (banking) were the most common methods used and were pursued mainly in the summer months (April to August). Total fish landings from the fishery were estimated at 1000 t year^-1, equivalent to about 10% of commercial catches for Trinidad. The principal species caught were kingfish, Scomberomorus cavalla (Cuv.), carite, Scomberomorus brasiliensis (Cuv.), cavalli, Caranx hippos (L.), snappers, Lutjanus spp., and groupers, Epinephelus spp. Many of the fishermen sold their catch to offset the costs of the fishing trip and thus competed with commercial fishermen. Recommendations for monitoring and possibly regulating the fishery through a licensing scheme are made.     

Modelling the impacts of removing seal predation from Atlantic salmon, Salmo salar, rivers in Scotland: a tool for targeting conflict resolution

Abstract  Bioenergetics were used to model the potential impacts on adult Atlantic salmon, Salmo salar L., stocks and rod fisheries of removing harbour seals, Phoca vitulina L., from three rivers of different scales in the Moray Firth, Scotland, viz: the Spey (large), Conon (medium) and Moriston (small). Overall, seals had the greatest impact on the Moriston, where removal of a single animal could increase cumulative catch by 17% during the fishing season. On the Conon and Spey the impacts were negligible, and resulted in increased catches of <1% annually. On all rivers eliminating seal predation had the greatest impact during the spring due to the smaller size of spring salmon sub-stocks. A generalised model of seal removal illustrated that stocks and catches increased by >33% in rivers with monthly rod catches ≤10 fish, but declined to <10% for rivers with catches >34 fish. The outputs of the models are qualitative, but provide a management tool for targeting action to resolve seal-salmon fishery conflict. Smaller salmon population units, and spring salmon sub-stocks and fisheries in particular, are most vulnerable to predation. The merit of this approach is discussed regarding the management of Special Areas of Conservation for salmon and seals.     

Fish catches among riverside communities around Lago Grande de Monte Alegre,Lower Amazon,Brazil

Forty‐six families living around the Amazon floodplain lake of Lago Grande de Monte Alegre, Brazil, were monitored between April 1993 and March 1995 with respect to their fish catches. The average catch per fisherman was 22 kg day^‐1. There were significant differences between subsistence and commercial fishing, and also across different seasons of the year. Catch was higher in the dry season and in the lake environment, followed by the river environment. The flooded forest was exploited in the rainy season. Out of a total of more than 70 fish species, 10 taxa comprised 75% of the catch. The catch in the Monte Alegre region concentrated on smaller species for local consumption. Prochilodus nigricans (Spix & Agassiz) was the most important species, followed by Liposarcus pardalis (Castelnau) and Plagioscion spp. Comparisons between quantities of fish consumed and fish marketed indicated that the fishery has a much more important role in the region than previously thought.     

Spatial and temporal differences in life history parameters of Rastrineobola argentea (Pellegrin, 1904) in the Lake Victoria basin in relation to fishing intensity

The small pelagic cyprinid, Rastrineobola argentea (Pellegrin), commonly known as dagaa, accounted for 60% of the total fish biomass and 40% of the commercial catches in Lake Victoria in 2015. However, some aspects of the biology of species (from which management interventions are based) have changed since 1970s; and yet harvest regulations have remained the same. In this study, spatial and temporal variations in life history traits of dagaa in the northern portion of Lake Victoria were examined in relation to fishing intensity to offer guidance on possible adjustments in managing the fishery. The mean standard length halved, whilst the length at 50% maturity (Lm₅₀) reduced by 27%, between the 1970s and 2015; however, the decline in Lm₅₀ was more pronounced in males than females. Data collected between 2014 and 2015 showed that immature individuals are largely harvested from inshore and mid‐island areas, whilst most of the fishes caught in open water areas are largely mature irrespective of the size of the gear used. The causes of the changes in these biological aspects, and the need for policy adjustment, are discussed in the context of changes in fishing pressure.     

Dynamics of floodplain fisheries in Bangladesh, results of 8 years fisheries monitoring in the Compartmentalization Pilot Project

Floodplain fisheries were monitored from 1992 until 2000 in the Compartmentalization Pilot Project in Tangail, Bangladesh. In permanent floodplains about 165 ± 28 kg ha^?1 of fish was caught annually. For seasonal floodplains, this figure was 83 ± 23 kg ha^?1 yr^?1. The fish catch exhibited a strong seasonal variation, with the highest catch in October, when the floodwater recedes towards the river, and the lowest catch during the dry season in April/May. The annual catch varied with the extent of flooding, with high catches in wet years and low catches in dry years. The extent of flooding was quantified through a Flood Index. Plotting the annual yields against this Flood Index provided a significant relation (P < 0.05), confirming the existence of a flood pulse. The fishing effort (f) and the catch‐per‐unit‐effort (CPUE) were significantly related (P < 0.05), whereby the fishing effort increased with increasing CPUE. The results are discussed within the frame of fisheries management in Bangladesh and highlight the need for long‐term data for proper evaluation of fisheries projects and the development of management schemes, and the difficulty of applying standard surplus production models in floodplain fisheries.     

Post-larval settlement of California spiny lobster Panulirus interruptus in Bahía Tortugas,Baja California and its relationship to the commercial catch

Based on data series for 8 consecutive years of settlement of post-larvae of the California spiny lobster Panulirus interruptus, its relationship to commercial catch was examined. The purpose was to establish a database to build a model for prediction of commercial catches of this lobster. The study area is a rocky shelf southwest of the bay entrance of Bahía Tortugas in Baja California Sur. A set of artificial collectors was placed at this site to obtain postpuerulus and early juveniles settlement for each month. Different simple regression analyses were performed on annual series of mean rates of post-larval settlement vs. commercial catches by the Bahía Tortugas fishing cooperative. We tested several delay periods between the series (4–8 years to approximate the age of fishery recruitment) to find the best-fit relationship. The relationship with a 5-year delay showed the highest positive correlation (0.7925) and was statistically significant. This delay period was used to regress the data for the years 1993–2001. The results showed that variations in the abundance of settled post-larvae in a given year are reflected in the commercial catch 5 years later and exhibit the same upward or downward trend. The one exception was the high post-larval settlement rate in 1997–1998, coinciding with the El Niño event, which was not mirrored by a similar increase 5 years later in the 2002–2003 catch. The pre-recruit index seems a very promising tool for predicting catches for this lobster fishery.     

Effect of supplementary stocking of juvenile brown trout,Salmo trutta,on yield in a Norwegian mountain reservoir

Abstract The effect of supplementary stocking of juvenile (age 0+), hatchery‐reared, brown trout, Salmo trutta L., on annual yields was assessed in a Norwegian mountain reservoir between 1979 and 2007. Fishing was mainly carried out by local fishermen with benthic gillnets. During the study period, annual stocking ranged from 0 to 52 500 fish (19.8 ha^?1). No stocking has been carried out since 1997. Annual yield varied from 1650 to 5653 kg, corresponding to 0.62–2.13 kg ha^?1. Exploitation rate in terms of number of gillnets and mean weight of 6+ fish (age when catchable size was reached) explained 64% of the variability in catches. Stocked fish contributed very little to the yield or catch‐per‐unit‐effort, exhibiting no positive correlation with stocking density. The lack of contribution from stocked fish was probably caused by a competitive bottleneck in the eroded epibenthic zone, causing high juvenile mortality. If stocking continues, it is recommended that fish with body lengths >15–20 cm are used.     

The use of at‐sea‐sampling data to dissociate environmental variability in Norway lobster (Nephrops norvegicus) catches to improve resource exploitation efficiency within the Skagerrak/Kattegat trawl fishery

Research into the influence of environmental variables on the behaviour of Norway lobster (Nephrops norvegicus), and hence catch rates, dates back to the 1960s (e.g., Höglund and Dybern, Diurnal and seasonal variations in the catch‐composition of Nephrops norvegicus (L.) at the Swedish west coast. ICES CM 1965/I46; Simpson, Variations in the catches of Nephrops norvegicus at different times of day and night. Rapport et Proés‐verbaux des Réunions Conseil permanent international pour 1'Exploration de la Mer 156:186). However, the use of fishery‐dependent data in identifying influential factors is relatively limited and only includes a number of papers on a limited dataset (e.g., Redant and De Clark, Diurnal variations in CPUE and length composition of the catches in a Nephrops directed fishery in the Central North Sea. ICES CM 1984/K:3; Maynou and Sardà, Influence of environmental factors on commercial trawl catches of Nephrops norvegicus (L.). ICES J. Mar. Sci. 58:1318). Here, we aimed to dissociate environmental variability in Norway lobster catches to improve resource exploitation efficiency within the Skagerrak and Kattegat trawl fisheries by utilising data collected as part of an extensive at‐sea‐sampling programme spanning 16 years. Catch rates were modelled using Generalized Additive Mixed Models (GAMMs) and considered a range of response variables, including depth, temperature, current speed, season, moon phase and time of day. The results obtained herein showed that time of day, season, depth, temperature, year, trawl type and location all significantly affect catch rates of Nephrops.     

Effects of mesh size,fishing depth and season on the catch and discards of short mackerel Rastrelliger brachysoma gillnet fishery at the mouth of Pattani Bay,Thailand

This study evaluates the impacts of mesh size of the net, fishing depth and season on the catches of target, non-target and discarded species for short mackerel Rastrelliger brachysoma gillnet fishery in the vicinity of Pattani Bay, Gulf of Thailand. The catches were examined for species composition, catch efficiency and size of catch. Samples were obtained monthly from February 2019 to February 2020 at three fishing depths (2.0, 4.0 and 6.0 m) using gillnets with three different mesh sizes (3.0, 4.0 and 4.5 cm). Including the target species, 112 species were collected, the majority of which were non-targeted fishes and crustaceans. The target species, Rastrelliger brachysoma, contributed only 6.0% by number and 8.76% by weight to the total catches. The 111 non-target species were classified into five categories based on potential sale prices. A total of 18 species, equivalent to 3.04% by weight of the total catches, were considered to be of negligible value and discarded or sold for the production of fish meal. Temporal variability was observed for number of individuals and biomass of total catches; the compositions of the catches clustered to two distinct seasons from December to March and from April to November, respectively. Spatial variability in this study focused on the depth profile which was found to significantly influence species richness and sizes of some fish species. For 12 species, mesh sizes of the gillnets significantly affected the number of individuals caught, as well as their weight and sizes. The novel findings of this study provide essential information that can be used for the sustainable management of the Rastrelliger brachysoma fishery in the Gulf of Thailand.

     

Biological implications of recreational shore angling and harvest in a marine reserve: the case of Cape Creus

• 1. Recreational shore fishing along the coast of the marine reserve of Cap de Creus (NW Mediterranean) was studied in 2007 and 2009 based on roving creel surveys (on‐site angler surveys during which anglers' harvests are examined by the survey clerk). The study aimed to assess the biological impacts of this leisure activity on coastal fish stocks and the potential risks arising from the use of exotic baits.
• 2. Recreational shore fishers employ seven different fishing techniques, of which the bottom fishing rod (a fishing rod whose hooks, together with the bait, lie on the bottom or near it by means of a heavy weight) is by far the most widely used method (nearly 90% of observations). In total, 25 fish species were identified in the catch from the bottom fishing rod.
• 3. The estimated annual shore fishing catches (c.3 tons) are much lower than those obtained by recreational boat and spear fishing (c.20 tons each), and those from commercial (artisanal) fishing (c.50 tons). The weighted mean vulnerability index and trophic level values in the catch from the bottom fishing rod are 52.2 and 4.03, respectively.
• 4. A minimum of 43% of the baits used by the shore anglers were live, non‐native species (mostly polychaetes).
• 5. Overall, results highlight the impact of shore angling on coastal fish communities of a protected area and the increasing environmental risks arising from the use of exotic marine baits, which constitute a potential and unregulated vector of introduction of non‐native species in the Mediterranean. Copyright © 2011 John Wiley & Sons, Ltd.

     

Threshold values of river discharge and temperature for anglers' catch of Atlantic salmon,Salmo salar L.

Abstract The catch by anglers of adult Atlantic salmon, Salmo salar L., was studied over a 5-year period in the River Gaula, Norway. Atlantic salmon were caught over a wide range (23–570 m³ s^?1) of the observed extent of river discharge (13–950 m³ s^?1) and throughout the range of temperature (4–23 °C), but both factors strongly affected catch rate. Significant correlations between the number of Atlantic salmon caught daily, and water temperature (r = 0.33) and river discharge (r = ?0.42) were found in 1987 and 1989, respectively. The highest daily catch occurred between 50 and 150 m³ s^?1, and at temperatures between 13 and 16 °C. Threshold values for water discharge and temperature were found to exist at 250 m³ s^?1 and 8 °C, with the highest catches below and above these values, respectively.     

Rapid invasion of a subtropical lake fishery in central Mozambique by Nile tilapia,Oreochromis niloticus (Pisces: Cichlidae)

• 1. Lake Chicamba, Mozambique (19°08′S; 33°08′E) is a large (116 km²) impoundment in the headwaters of the Buzi River system, which was invaded by Oreochromis niloticus in 1996 from a small (<0.3 km²) upstream reservoir.
• 2. Experimental and artisanal catch data showed no O. niloticus until January 1996; after this O. niloticus was recorded in up to 83% of experimental seine net catches, 33% of experimental gill net catches, 43% of boat angling and 23% of shore angling catches, and in 48% of artisanal gill net catches.
• 3. During the period January to March 1997, O. niloticus mean (upper, lower 95% confidence interval) yields in the artisanal fishery were 5.2 (3.6, 7.0) t month^?1.
• 4. The rapid invasion of this lake illustrates the significant invasion threat that small point‐sources of this species pose to southern African freshwater systems.
• 5. The study recommends: (1) that this species should not be used for aquaculture or fisheries enhancement in catchments that have not been invaded, and (2) that the eradication of potential point sources of O. niloticus in non‐invaded catchment systems should be considered.

Copyright © 2007 John Wiley & Sons, Ltd.     

东海带鱼渔获量对捕捞压力和气候变动的响应

为了解捕捞压力和气候变动对东海带鱼渔业产量的共同影响,对1956-2006年东海带鱼渔获量进行了分析.东海带鱼渔获量时间序列可划分成变化趋势和年间变动.变化趋势主要归因于捕捞努力量的单调增长,Fox模型拟合结果,东海带鱼渔获量与捕捞努力量关系显著(P＜0.01).移除趋势后,其年间变动与陆地降水、季风风速、海表水温和热带气旋影响指数相关(P＜0.05).结果表明,气候变动影响到东海带鱼渔获量年间变动:陆地降雨和径流携带大量营养盐进入沿岸生态系统;而季风则驱动营养盐的扩散和循环,影响着营养盐的利用效率;水温的升高不仅有利于带鱼的性腺发育与成熟,还能增加带鱼的饵料供应;热带气旋所形成的水团流动、风生混合、上升流等能促进营养盐供应并增加水域的生物量.从捕捞努力量和气候变量拟合东海带鱼渔获量的结果来看,与实际渔获量显著相关(P＜0.01),并且能够很好地反映出实际带鱼渔获量的变化趋势和年间变动.这说明渔获量的变化受到捕捞效应和气候变动的双重影响,未来气候变化将有利于东海带鱼渔业产量的增加,且渔获量年间变动幅度将会比以往更大.     

Trawl catch composition in relation to Norway lobster (Nephrops norvegicus L.) abundance on the Farn Deeps grounds, NE England

Management of fisheries directed at a particular species (the target) is often complicated by the capture of other species (the bycatch), each of which may be subject to competing management objectives. Optimal management depends upon a good understanding of how catches of the target species vary in relation to bycatch of other species of commercial and conservation concern. This paper considers the composition of trawl catches taken on a Norway lobster (Nephrops norvegicus) ground off the NE coast of England, examining the factors that determine catch rates of Nephrops and various commercial finfish species taken as bycatch. We describe the results of a trawl survey undertaken using four commercial vessels deploying standard commercial gear. Only about 19% of the variance in Nephrops catch rates could be accounted for by variations in local density, as measured independently by underwater television survey of Nephrops burrows. Variations in Nephrops catch rates were dominated by changes over time, particularly in relation to tidal state. Five commercial bycatch species were taken alongside Nephrops in significant quantities. Catch rates of haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus) and lemon sole (Microstomus kitt) varied independently of Nephrops, whereas catches of cod (Gadus morhua) and plaice (Pleuronectes platessa) were inversely related to Nephrops. We conclude that it is possible for the commercial Nephrops fishery to use spatial and temporal targeting to maximise catches of N. norvegicus whilst minimising the bycatch of some fish species. If catch statistics are used to identify Nephrops-directed fishing métiers for fishery management purposes it is vital that data are examined at the level of individual hauls or fishing trips. Aggregation of data at a higher level risks obscuring the effectiveness with which the fishery is targeted at Nephrops.     

A review of seed production and stock enhancement for commercially important portunid crabs in Japan

We reviewed the present status of seed production for stock enhancement and evaluated the impact of stocking on commercial catches in Japan, of portunid crabs particularly Portunus trituberculatus and Scylla paramamosain. The mean survival rate from hatching to first-stage crabs was around 10%, and 20–30% of the larval culture trials conducted in recent years could not harvest juveniles during seed production. To achieve reliable mass seed production technologies, measures for controlling disease and the nutritional condition of larvae in seed production tanks are required to be developed. The main spawning season extends from April/May to July. Reflecting their life cycle characteristics such as high growth rates, crabs recruit to the fishery after September and largely contribute to the commercial landings until December in the hatching year. The main release season of juveniles is from June to July. Consequently, released juveniles are expected to contribute to commercial landings in the release year. Analyses of catch and release statistics in two small bays estimated the yield from released individuals (YPR, yield per release) at 2.4 g for P. trituberculatus and 3.3–7.7 g for S. paramamosain, which were similar to values reported from tagging surveys. On a major regional basis, catch and release histories of P. trituberculatus highlighted the impact of hatchery releases on commercial landings as 33.6 g YPR in the Seto Inland Sea, where catches and releases have been greatest. Although the YPR estimates were different between small bays and major regional seas, Japanese stock enhancement programmes should have had an impact on portunid crab production, dependent on the magnitude of the releases.     

Coupling an oceanographic model to a Fishery Observing System through mixed models: the importance of fronts for anchovy in the Adriatic Sea

Anchovy, Engraulis encrasicolus, forms the basis of Italian small pelagic fisheries in the Adriatic Sea. The strong dependence of this stock on environmental factors and the consequent high variability makes the dynamics of this species particularly complicated to model. Weekly geo‐referenced catch data of anchovy obtained by means of a Fishery Observing System (FOS) from 2005 to 2011 were referred to a 0.2 × 0.2 degree grid (about 20 km²) and associated with the environmental parameters calculated by a Regional Ocean Modelling System, AdriaROMS. Generalized Additive Mixed Models (GAMM) with and without random effects were used to identify a relationship between abundance in the catch and oceanographic conditions. The outcomes of models with no random effects, with random vessel effects and with the random vessel and random week‐of‐the‐year effects were examined. The GAMM incorporating a random vessel and week‐of‐the‐year effect were selected as the best model on the basis of the Akaike information criteria (AIC). This model indicated that catches (abundance) of anchovy in the Adriatic Sea correlate well with low temperatures, salinity fronts and sea surface height, and allowed the identification of areas where high concentrations of this species are most likely to occur. The results of this study demonstrate that GAMM are a useful tool to combine geo‐referenced catch data with oceanographic variables and that the use of a mixed‐model approach with spatial and temporal random effects is an effective way to depict the dynamics of marine species.     

An evaluation of the Leslie—DeLury method and of a weighted method for estimating the size of a closed population

A modification of the Leslie—DeLury method for estimating the size of a closed population is presented, and the precision and biases of both the Leslie—DeLury method and the modified method are examined. The modification involves an iterative procedure based on weighted linear regression of the catch-per-unit-effort upon the corrected-cumulative-catch. The standard deviations of the predicted initial population sizes obtained using this weighted method are found to be on the average, sixteen percent smaller than those obtained using the conventional corrected Leslie—DeLury method. Both of these methods have small positive biases which are approximately equal on the average, to one-fifth of the standard deviation of the predicted initial population size. Hence, the weighted method is on the average, both more precise and less biased.The conclusions stated above are based on the results of a simulation model. Simulations were done within the following ranges of parameter values: 2000 N₁ 12000, 1 N_c 15, 0.5 (1 − Exp(−Mf̄q̄)) 0.9, 0.0 < S_f/f̄ 1.0, 20 M 8,

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where N₁ is the initial population size, is the mean effort expended per sampling interval, S_f is the standard deviation of the efforts, M is the number of sampling intervals, is the mean catchability, S_q is the standard deviation of the catch-ability, and N_c is clump size for aggregated catches. The mean catchability is treated as a constant rather than a parameter because it can be set to any specified value by the appropriate selection of the units of measurement of effort.The suggested weighting is developed for catch data which are obtained by random sampling, without replacement, from a closed population (a population in which there is no immigration, no emigration, no natural mortality, and no recruitment) for which there is no interference among the units of effort, and for which the measurement errors in the catches and efforts are negligible. The catchability and the effort are treated as random variables. Aggregation of the catches is allowed by dividing the population among a number of clumps (each of the same size) which are subject to capture.The weights are computed using the equation (

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is the effort expended in interval i, C_i is the catch from interval i, and where the coefficients a and b are obtained from the regression of the squared residuals using a quadratic model with no constant term (( ). The predicted catches, , are obtained from the weighted regression of the catch-per-unit-effort upon the corrected-cumulative-catch (done in the previous iteration). The initial estimates of the predicted catch are obtained using the conected Leslie—DeLury estimate. Five to ten iterations are generally needed to obtain stable estimates.     

Factors influencing the abundance of three common nearshore fishes in south‐west Lake Michigan

Variation in the distribution and abundance of nearshore fishes is critical to understand food web processes and fishery management issues in Lake Michigan. This study characterised patterns in abundance of three common nearshore species, yellow perch Perca flavescens (Mitchell), round goby Neogobius melanostomus (Pallas) and alewife Alosa pseudoharengus (Wilson), in relation to spatio‐temporal, abiotic and biotic factors using gillnet sampling conducted across 5 years at multiple locations representing different substrates. Significant variations were observed in alewife and round goby catches between locations. A negative relationship between round goby and age‐0 yellow perch catch was observed, indicating potential competition between the two species. This study demonstrates that variability in nearshore fish communities can be driven by factors including substrate and interspecific interactions. Given the prominent role these species play in Lake Michigan's food web and thus their importance to fishery production, a thorough understanding of these factors is warranted.