Mixed-species Fisheries Research Articles

One mesh does not fit all: A dual compartment codend provides flexible selectivity opportunities to manage mixed fisheries

Fisheries management is moving from a single-species to an ecosystem-based approach to better balance the technical, biological, and economic aspects of the many mixed-species fisheries in the world. Most mixed fisheries are conducted with trawl gears, which have a hereditary challenge of providing a species and size selectivity that meets the development in management objectives of reducing unwanted catch. When fishing on sympatric species, a codend that allows separation of species into different compartments during towing can provide separated catch fractions with contrasting selectivity opportunities. In a case study, we quantified the processes of vertical separation and length-based selectivity for a dual compartment codend having an upper, large mesh compartment intended for cod (Gadus morhua) and a lower, small mesh compartment intended for Nephrops (Nephrops norvegicus). This allowed identification of which processes to improve to reduce unwanted catch. The gear concept delivered a complex selectivity profile while enabling high flexibility to adjust selectivity, also at sea, and provides a tool for the industry to ensure better compliance with increasing management ambitions. An unusual selectivity curve for cod resulted from the contrast in mesh size between the two compartments. Cod and Nephrops smaller than and just above the minimum conservation reference size (MCRS) were efficiently released through the 120 mm square meshes in the upper and the 60 mm square meshes in the lower compartment, respectively. Cod (47%) had a low probability of escaping when caught in the lower compartment, and Nephrops (9%) were lost when caught in the upper compartment. A compartment in the dual compartment codend may easily be changed at sea to adjust the selectivity according to the mix of species encountered, available quota portfolio, and management regime.

Recovering highly exploited stocks with heterogeneous resilience capacities in mixed fisheries under uncertain illegal fishing

The complexity of stock recovery strategies in mixed-species fisheries involving heterogeneous resilience capacities of target and incidentally harvested species is further challenged by possible levels of illegal exploitation rates. This context adds to the uncertainty for decision-makers when establishing proper management measures to recover highly overexploited stocks towards the selected target reference point (e.g., BiMSY). With a dynamic bioeconomic model, we analyzed the stock recovery performance of a mixed fishery with two overexploited species using combinations of moratoria durations (μi) and subsequent exploitation rates (FiMSY), under possible states of nature of illegal fishing (θj). Alternative stock recovery management strategies (Dj) were considered to include the effect of possible θj. Both stocks present heterogenous abundance, renewability capacities, and relative prices. The impact of possible levels of illegal fishing on the legal resource rent and the corresponding forgone legal resource rent was also assessed. A Monte Carlo analysis was undertaken to calculate the risk of falling below BiMSY considering alternative management strategies to deal with possible illegal fishing (ρi,Dj). Overexploited species with lower resilience capacity require longer moratoria timelines, extended when dealing with alternative states of nature of illegal fishing. Management strategies based on target species achieved the highest fishery legal resource rent while having the highest risk of not recovering the stock of incidental species with lower resilience capacity. In this study, a mixed-strategy management decision where a moratorium is based on target species and subsequent exploitation rates (Fi) based on lower resilience species resulted in minimum risks of falling below species MSY at high levels of illegal fishing. This same strategy resulted in maximum NPV of resource rent per vessel for alternative levels of illegal fishing. Lack of consideration of possible levels of illegal fishing may lead to failure in achieving stock recovery to target reference points in mixed fisheries.

Trade-offs between physical risk and economic reward affect fishers’ vulnerability to changing storminess

Climate change-driven alterations in storm frequency and intensity threaten the wellbeing of billions of people who depend on fisheries for food security and livelihoods. Weather conditions shape vulnerability to both loss of life and reduced fishing opportunities through their influence on fishers' daily participation decisions. The trade-off between physical risk at sea and the economic rewards of continued fishing under adverse weather conditions is a critical component of fishers’ trip decisions but is poorly understood. We employed a stated choice experiment with skippers from a temperate mixed-species fishery in southwest England to empirically assess how fishers trade off the risks from greater wind speed and wave height with the benefits of expected catch and prices. Technical fishing and socio-economic data were collected for individual fishers to identify the factors influencing trade-off decisions. Fishers preferred increased wind speed and wave height up to a threshold, after which they became increasingly averse to worsening conditions. Fishing gear, vessel length, presence of crew, vessel ownership, age, recent fishing success and reliance on fishing income all influenced the skippers’ decisions to go to sea. This study provides a first insight into the socio-economic, environmental, and technical fishing factors that can influence the sensitivity of individual fishers to changing storminess. These insights can help to inform fisheries climate vulnerability assessments and the development of adaptation measures.

Shark discards in selective and mixed-species pelagic longline fisheries.

The conservation status of several pelagic shark species is considered vulnerable with declining populations, yet data on shark fishing mortality remain limited for large ocean regions. Pelagic sharks are increasingly retained by mixed-species fisheries, or are discarded and not reported by selective fisheries for tunas (Thunnus spp.) or swordfish (Xiphias gladius). We estimated the fishing mortality of sharks (landings plus discard mortalities) in a South African-flagged pelagic longline fishery with diverse targeting and discard behaviour. A hierarchical cluster analysis was used to stratify the fleet according to the relative proportions of tunas, swordfish, blue sharks (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) landed by individual vessels between 2013 and 2015. A spatial analysis of logbook data indicated that subfleets operated in distinct fishing areas, with overlap. Approximately 5% of all commercial longlines set during 2015 were sampled by a fisheries-independent observer, and the species, discard ratios and physical condition at discard of 6 019 captured sharks were recorded. Blue sharks and shortfin makos dominated observed shark catches, which were comprised of nine species and two species groups. Some 47% of observed sharks were retained and 20% were discarded in good physical condition. Only 4% of shortfin makos were discarded, compared to 68% of blue sharks. Blue shark discard mortality rates were twice as high as published at-vessel mortality rates, suggesting that onboard handling, among other factors, contributed to discard mortalities. Extrapolation to total fishing effort indicated a near 10-fold increase in blue shark and shortfin mako fishing mortality compared to an earlier study (1998-2005). Escalating shortfin mako fishing mortality was attributed to increased targeting to supply higher market demand. Discarding of blue sharks by selective fishing for tunas and swordfish had a greater impact on their fishing mortality than retention by shark-directed fleets. Higher levels of observer sampling are required to increase confidence in discard ratio estimates.

Artificial light improves escapement of fish from a trawl net

AbstractThe elimination of unwanted catch in mixed species fisheries is technically challenging given the complexity of fish behaviour within nets. Most approaches to date have employed technologies that modify the nets themselves or use physical sorting grids within the gear. There is currently increasing interest in the use of artificial light to either deter fish from entering the net, or to enhance their escapement from within the net. Here, we evaluated the differences in catch retained in a standard otter trawl, relative to the same gear fitted with a square mesh panel, or a square mesh panel fitted with LEDs. We found that the selectivity of the gear differed depending on water depth. When using a square mesh panel in shallow depths of 29–40 m the unwanted bycatch of whiting and haddock was reduced by 86% and 58% respectively. In deep, darker water (45–95 m), no change in catch was observed in the square-mesh panel treatment, however when LEDs were added to the square-mesh panel, haddock and flatfish catches were reduced by 47% and 25% respectively. These findings demonstrate the potential to improve the performance of bycatch reduction devices through the addition of light devices to enhance selectivity. The results also highlight species-specific and site-specific differences in the performance of bycatch reduction devices, and hence a more adaptive approach to reduce bycatch is probably required to maximize performance.

Meta-analysis of Queensland’s coastal Indigenous fisheries: Examining the archaeological evidence for geographic and temporal patterning

Marine fisheries have been a critical part of Aboriginal and Torres Strait Islander people’s connection to land and sea country in Queensland, Australia for millennia. However, no archaeological studies have examined regional variability in the role of fish within subsistence regimes or the distribution of targeted fish species throughout the Holocene. We utilised a meta-analysis approach to conduct the first comprehensive assessment of Indigenous fisheries along the eastern Queensland coast. Data from 44 archaeological sites were grouped according to marine bioregion to facilitate broad comparison between sites across the study area. These sites were predominantly associated with mid-to-late Holocene occupation, and provided an assemblage of 45,052 recovered fish bones, of which 6606 were identified most commonly to family-level. Results indicate clear geographic patterning in the ubiquity of fish species captured, and for some marine bioregions an increase through time in the range of species targeted. Archaeological data indicate mixed species fisheries, with a complex range of habitats and diverse fish species harvested by people in relative proximity to the sites. These harvesting decisions were mediated by local ecological knowledge, awareness of fish behaviour, and cultural preference for certain species. These outcomes support existing models for the region, which document a shift in subsistence regimes during the mid-to-late Holocene, particularly an increased reliance on marine resources and expansion in diet breadth. Future research needs to address geographic gaps in data availability and implement globally recognised ichthyoarchaeological quantification and identification protocols to comprehensively examine geographic and temporal variability in Queensland’s Holocene Indigenous fisheries, and contribute to regional models of long-term subsistence change.

Relative mobility determines the efficacy of MPAs in a two species mixed fishery with conflicting management objectives

Marine Protected Areas (MPAs) have been used to protect species in need of conservation and as a fisheries management tool. It has been suggested MPAs can benefit mobile stocks by protecting spawning grounds whilst also allowing yields to be maintained as mature fish move out of the protected areas. However, the robustness of this claim in mixed species fisheries has yet to be established. We use a simulation model to explore the efficacy of spatial closures and effort regulation when other forms of fishery control (e.g., Total Allowable Catches) are absent or non-enforced as ways of addressing management objectives that are difficult to reconcile due to the contrasting life-histories of a target and a bycatch, conservation species in a two-species fishery. The mobility of each stock in such a fishery affects the benefits conferred by an MPA. The differing management objectives of the two species can be partially met by effort regulations or closures when the species exhibit similar mobility. However, a more mobile conservation species prevents both sets of aims being met by either management tool. We use simulations to explore how spatial closures and effort regulation can be used to seek compromise between stakeholders when the mobility of one stock prevents conflicting management objectives to be fully met. Our results demonstrate that stock mobility is a key factor in considering whether an MPA can meet conflicting aims in a multispecies fishery compromised of stocks with differing life histories and mobilities.

Effects of environmental data temporal resolution on the performance of species distribution models

Abstract Assessing how environmental changes affect the distribution and dynamics of animal populations is becoming increasingly important, enabling ecologists to better predict the effects of global warming, biodiversity reduction or habitat degradation. In this context, the use of remotely sensed and modeled environmental variables play a key role by providing spatially contiguous predictors for species distribution models. Nevertheless, little attention has been paid to how the temporal scale of the environmental predictors could affect predictions. Here, we investigate how different levels of temporal integration of environmental variables influence model performance. We present the case study of the distribution of the three most discarded species of the bottom trawl mixed-species fishery in the Cantabrian Sea from 2004 to 2008. We find clear improvements in model prediction performance when using environmental data aggregated by season, while the estimation of the relationship between species and their habitats is better represented by time-specific data. Our results also support the hypothesis that medium and lived species are more sensitive to seasonal environmental conditions, in contrast to results found for long-lived marine mammals and seabirds, indicating that their distributions are better predicted by long-term mean conditions.

Selecting a subset of the commercial catch data for estimating catch per unit effort series for ling (Molva molva L.)

Ling is an important species for the Norwegian longline fishery. Motivated by an apparent steep decline in a catch per unit of effort (CPUE) series for ling based on commercial longline data during the periods 1971–1993 and 2000–2003, ling was classified as Near Threatened and placed on the Norwegian Red List, which caused difficulties for the marketing of ling. To examine the validity of the conclusion that this CPUE series indicated that the ling stock was Near Threatened, we estimated CPUE series for the period 2000–2012 based on extensive logbook data available since 2000. The Norwegian longline fishery is a mixed-species fishery: therefore, we constructed three different CPUE series based on; (1) all catches including zeros, (2) selected catches that appeared to have target ling and (3) selected vessels for each year that seemed to have often targeted ling during that year. It was concluded that these CPUE series indicated that the abundance of ling in Norwegian waters has been fairly stable or increasing since at least 2000, and hence there was no compelling evidence that the abundance of ling was declining. Based on our analysis, ling was removed from the Norwegian Red List. Even though all three series indicated increasing abundance, their precision varied, and estimates of how much the ling stock has increased depended on the subset of the commercial data on which the CPUE series was based.

Physiological impairment of adult sockeye salmon in fresh water after simulated capture-and-release across a range of temperatures

Sockeye salmon abundance in the Fraser River has declined precipitously over the past two decades, reducing fishing opportunities for this ecologically, culturally and economically valuable species. Release of non-target species that are captured incidentally is a growing conservation measure used in managing mixed species fisheries. Fraser River sockeye salmon are released from commercial, First Nations, and recreational fisheries, however little research exists investigating the fitness and fate of released sockeye salmon, and none examine how the effect of ecologically relevant temperature ranges interact with capture stressors. We exposed adult migrating summer-run sockeye in freshwater to simulated capture and release stressors, including exhaustive exercise and air exposure, at temperatures spanning the range of historic, current and predicted future migration conditions in the Fraser River (13 °C, 19 °C, and 21 °C) to understand the physiological consequences of these stressors acting in synergy. Three minutes of exhaustive exercise significantly raised the plasma lactate, chloride, sodium, and osmolality, and lowered the plasma potassium of these fish, and one minute of air exposure following exercise elevated glucose (females only) and exacerbated high lactate levels. The inability of all air-exposed fish in the warmest temperatures to maintain equilibrium upon return to the water is evidence of the effects of warm temperatures. Fewer than half of the air-exposed fish in the coolest temperature demonstrated this impairment. Further, air exposure resulted in a marked depression of ventilation rates in the warmest temperatures. This is the first experiment investigating the physiological disturbance of capture and release at various temperatures on adult Pacific salmon, and the results suggest a need for better understanding of the potential fitness consequences to improve the management of this important resource.

Development of a codend concept to improve size selectivity of Nephrops ( Nephrops norvegicus) in a multi-species fishery

Species-specific differences in morphology and minimum landing size will often result in high discard rates in mixed-species fisheries. This is the case in the Danish Nephrops-directed fishery and the aim of this study was to improve the size selectivity of Nephrops without increasing catch below minimum landing size (MLS) of cod ( Gadus morhua) and plaice ( Pleuronectes platessa). A new codend concept combining square meshes and diamond meshes in a four-panel configuration was developed and tested in the Kattegat and Skagerrak. A 5 m long 70 mm square-mesh panel was inserted in the lower panel of the codend to increase size selectivity of Nephrops while the remaining three panels were made of standard 90 mm diamond-mesh netting to maintain selectivity of plaice and cod. The concept succeeded in significantly ( p < 0.0001) increasing L 50 of Nephrops without increasing discards of either plaice or cod. When using the new codend concept, expected numbers of Nephrops below MLS (carapace length = 40 mm) were reduced by approximately 37% but the expected weight of marketable catch was also reduced by 21%. The concept was adjusted and tested in a second experiment so that fishing using the new net configuration would remain commercially viable. This experiment indicated that size selectivity of Nephrops could be adjusted by changing the square-mesh panel but differences in mean selection between the gears were insignificant. We introduce a new analytical approach with increased statistical power to detect differences in selection parameters between different codends.

Rebuilding depleted fish stocks: the good, the bad, and, mostly, the ugly

Abstract Murawski, S. A. 2010. Rebuilding depleted fish stocks: the good, the bad, and, mostly, the ugly. – ICES Journal of Marine Science, 67: 1830–1840. Recovery of depleted fish populations has become an important theme in national and international negotiations and commitments regarding sustainability. Although up to 63% of fish stocks worldwide may be in need of rebuilding, only 1% are currently classified as “rebuilding”, and fewer yet have been “rebuilt”. Recent history in stock recovery provides a rich source of examples of rebuilding plans across a spectrum of execution (“good”, “bad”, “ugly”, and “in progress”). Of 24 depleted stocks with formal plans that successfully reduced the fishing mortality, all but one exhibited signs of recovery. The median instantaneous annual rate of biomass recovery (0.16) was similar to the rate of depletion (−0.14) experienced, but stocks with more vulnerable life histories recovered substantially slower than they had been depleted. Most successful rebuilding programmes have incorporated substantial, measurable reductions in fishing mortality at the onset, rather than relying on incremental small reductions over time. A particularly vexing issue is the differential pace of recovery among relatively productive and unproductive components of mixed-species fisheries. Rebuilding the majority of stocks classified worldwide as “overfished” will take a more effective, consistent, and politically supported stock-recovery paradigm, if society is eventually to meet its articulated sustainability goals for global fisheries.

Seasonal distribution of orange roughy ( Hoplostethus atlanticus) on the Porcupine Bank west of Ireland

The Porcupine Bank west of Ireland comprises broad, gently sloping areas interspersed with complexes of hills and canyons. An Irish deepwater fleet has exploited orange roughy ( Hoplostethus atlanticus) here since 2000. Seasonal aggregations are targeted on hills and a year-round bycatch is taken in a flat ground mixed species fishery, with roughy being caught at 675–1450 m depth. Orange roughy ( N = 3575) were sampled at sea in both of these fisheries from August 2003 to April 2004 and during February 2005. Standard length (SL) and stage of sexual maturity were recorded, with adult fish varying between 29 and 60 cm SL. Spawning of orange roughy extended from January to April and peaked in late February to early March in both 2004 and 2005. Mean SL/haul for orange roughy differed significantly between the spawning season (January–April) and the rest of the year on each of ‘flat’ and ‘hill’ habitats. Mean SL/haul also differed between ‘flat’ and ‘hill’ during the spawning season but not during the rest of the year (Mann–Whitney tests, P < 0.05). Proportions of orange roughy >30 cm SL/haul that would not spawn in the current year did not differ between ‘hill’ and ‘flat’ during the spawning season (Mann–Whitney test, P = 0.07) but spawning ogives for the two habitats were significantly different ( F = 18.81, d.f. = 2.78, P < 0.01) and provided a lesser L 50 for roughy on the ‘hills’ (34.10 cm SL) than on the ‘flat’ (37.00 cm SL). Hill aggregations appeared to comprise larger fish with a higher probability of spawning in the current year and hence seasonal migration of fish from flat ground to spawning aggregations on the hills is suggested.

Studies On The Atalla Fishery Of The Lower Anambra River, Nigeria

The atalla lift net fishery of the lower Anambra river was investigated from May, 1998 to March, 1999 by sampling commercial fishers' catches and obtaining from fishers information relating to fishing time, catch, income and preservation of catch. The species composition of the total catch showed that there were 40 species of fin fish, two decapod crustaceans and a mollusc. Pellonula leonensis was the most abundant fish species by number and weight followed by Parailia pellucida. P. leonensis and P. pellucida exhibited inverse numerical relationship. The target clupeids and schilbeids constituted over 68% and 55% by number and weight respectively of the overall catch. A total of 23 atalla fishing units operated in the lower Anambra river and over 91% of them were operated by Ijaw migrant fishers. The net of atalla was made up of two ply and 10 mm mesh size or plastic mosquito gauze. The canoe was planked and there was a close relationship between the base of the atalla lift net and the length of the canoe (r2=0.91). The total annual catch was estimated at 155 t excluding the quantity consumed by the fishers' families. Smoking was the main method of preservation. It took 2±0.04 days to dry a batch of elem and smoked fish were packaged in small baskets lined with cement paper. Atalla fishery appeared not to be detrimental to the fisheries of the relatively large-sized fish species and it should be encouraged to harvest the abundant clupeids and schilbeids in the mixed species fishery of the lower Anambra river. Mesh regulation is not relevant in the management of atalla fishery. Key Words: Atalla fishery, catch, species composition, abundance, preservation, Anambra river Bio-Research Vol.2(1) 2004: 82-90

Objective methods for defining mixed-species trawl fisheries in Spanish waters of the Gulf of Cádiz

This paper presents the results obtained from applying an objective classification method to the daily commercial landings data of the Spanish bottom-trawl fishery off the Gulf of Cádiz for defining mixed-species fisheries or fishing trip types. Definition of these operational fisheries was based on the relatively unique and homogeneous species composition of the landings and technical characteristics of the vessels. The method was based on cluster (CA) and linear discriminant (DA) analysis techniques. The application of CA techniques to the 1993 data matrix of daily catch per unit of effort per species and active vessel resulted in the identification of 22 types of fishing trips, which were defined by the relative importance of their target and accessory species. Subsequently, different linear discriminant functions were derived for each FTT in order to classify new records. Five different fleet types were also identified through CA techniques according to the vessel’s length, GTR and HP. The distribution of active vessels by fleet type in each landing port showed a great local and regional heterogeneity of the fleets exploiting the multispecies fishery. A correspondance analysis between types of fleets and fishing trips showed a high correlation, indicating the existence of a direct relationship between the capacity of vessel mobility and the bathymetric situation of the fisheries.

A comparison of three indices of fishing power on some demersal fisheries of the North Sea

The scope of this study is to identify temporal dynamics in fishing power, by deriving three different indices (IFP1, IFP2, IFP3) based on three independent methods. IFP1 is derived from the GLM analysis of the relationship between fishing mortality and fishing effort, assuming that total fishing mortality estimates from XSA (eXtended Survivors Analysis) are accurate. IFP2 is derived from the GLM analysis of the difference between the Log-CPUE of a vessel and the average Log-CPUE of a set of reference vessels, which are chosen with regards to the stability of their Log-CPUE over time. IFP3 is derived from the GLM analysis of the Log-CPUE of a vessel relative to some external survey abundance index. Particular attention is paid to the horsepower and year effects in IFP1, IFP2, and IFP3. This methodology is applied to the Danish, Dutch, English and Norwegian demersal fisheries of the North Sea. The fishing power estimated by all indices increases with horsepower, particularly in relation to target species. Despite less consensus in the estimation of annual variations in fishing power, some important features are highlighted. First, there are cases where fishing power has consistently increased over the period of investigation, possibly through an overall increase in fishing efficiency. Second, there are examples where fishing power has increased relative to one species, and remained constant or even decreased in relation to another one. In the context of mixed-species fisheries, this feature might reveal a shift in fishing tactics. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Measuring contributions to economic production—use of an Index of Captured Ecosystem Value

The production of an economic good derived from a renewable natural resource base involves the extraction of ecosystem function values as represented by the contribution made to production by the originating ecosystem. The artisanal mixed-species fisheries of Jamaica is used as a case study in the development of a biophysically based index to account for captured ecosystem values (or embodied ecosystem values) and an examination of the extent to which those values are proportionately reflected in monetary exchange values. The Index of Captured Ecosystem Value (ICEV) is developed from a basis in information theory relevant to an analysis of network flows in ecosystems. Technical coefficients, describing the production relationship between ICEV values and market values of catches associated with individual fishing efforts in Jamaican fisheries, reveal that captured ecosystem function associated with fisheries using distinct technologies (i.e. China net, trap, hand line, palanca and speargun) are valued differently by the market. This surplus value is rooted in the observation that certain fisheries target species that are more connected within the coral reef food web than those species typically captured by other fisheries. Consideration of the biophysically based contributions of coral reef ecosystems to fisheries production reveals distortions between market and supply-side values, indicating that the role of ecosystems is not being consistently treated. Comment and direction is offered regarding the development of indices of ecosystem function or value that can be applied to policy questions concerning the extraction of a renewable natural resource.

Estimation of technical interactions due to the competition for resource in a mixed-species fishery, and the typology of fleets and métiers in the English Channel

In a multi-gear and multi-species artisanal fishery, the level of technical interactions (i.e. the competitive externalities resulting from a shared exploitation of common resources or fishing grounds) among various fishing units is high. Assessing these technical interactions is of great importance for fishery management, as any control applied to one fishing unit may have positive or negative effects on others. The magnitude and direction of these effects cannot be easily measured, unless all fishing units and species in the fishery are considered simultaneously. Technical interactions are particularly important in the complex artisanal fisheries of the English Channel. Using a bioeconomic model of the English Channel that incorporates all the major fishing units (the BECHAMEL model), we describe a method for measuring and classifying the technical interactions due to the competition for resource (stock externalities). The results are used to develop a typology of métiers and fleets based on their overall level of interaction for the resource. We also define fleets and métiers as structuring, dependent, intermediate or autonomous.

Biological Bases for Mixed-Species Fisheries: Species Co-distribution in Relation to Environmental and Biotic Variables

Factors influencing the degree of temporal and spatial co-occurrence of seven important demersal fish species on Georges Bank (Northwest Atlantic) were analyzed. Species/age co-occurrence data from a 24-yr time series of research vessel bottom trawl surveys were evaluated in relation to temperature and depth preferences, spatial distribution, and species/age abundance trends. Co-occurrence of various species/age groups varied greatly by season and was significantly correlated with overlap in temperature/depth preference indices and annual species abundance. Although species abundance and proportion of survey tows having particular species varied widely over the time series, trends in spatial dispersion indices of particular species were not generally related to abundance. Multiple regression models incorporating temperature and depth preference overlap and species abundance as independent variables accounted for a high proportion of total variance in the transformed proportion of survey tows containing co-occurring species (mean R2 = 0.69 for age 1 + fishes, mean R2 = 0.44 for age 0). Results indicate that although the frequency of technological (bycatch) interaction among pairs of species may be highly variable depending on season, area, and year, co-occurrence among species is nonetheless relatively predictable. Minimization of the potential adverse impacts of mixed-species harvesting (i.e. failure to achieve management goals for all species simultaneously) may be accomplished by exploiting stocks individually during periods of maximum spatial separation and by using information on the determinants of species co-distribution to more accurately forecast bycatch levels.

Optimal Effort Allocation Among Competing Mixed-Species Fisheries, Subject to Fishing Mortality Constraints

A linear programming (LP) approach to effort allocation among two or more fisheries (fleets) exploiting several common species/stocks is described and applied to otter trawl fisheries exploiting demersal fish stocks on Georges Bank (northeastern United States). Total instantaneous fishing mortality on a particular species (i) is computed as the linear summation of fishing mortalities generated by each fishery (j):[Formula: see text]where fj is the amount of standardized fishing effort exerted in fishery j and qij is the catchability coefficient for species i taken in fishery j. Mortality on species i due to both directed fishing and by-catch can thus be accounted for in the qij's. Optimal allocation of effort among the j fisheries may be considered a minimization problem (minimize Σfj), subject to the constraints that fishing mortality rates on particular species are maintained at, above, or below certain predefined levels. Fishing mortality goals for individual species can be based on various biological and/or economic criteria: fishing mortality rates that prevent growth or recruitment overfishing, or that optimize productivity from predator–prey systems. Other constraints in the LP model may be included to modify optimal solutions based on various economic and social considerations (e.g. protection of certain fisheries). Sensitivity analyses indicate the general infeasibility of maintaining relatively high or low fishing mortality rates on ubiquitously distributed species, while moderately fishing species with more discrete distributions, due to by-catch considerations.