Threshold management strategies for exploited fish populations

Thesis (Ph.D.) University of Alaska Fairbanks, 1994Under a threshold management strategy, harvesting occurs at a constant rate but ceases when a population drops below a threshold. The threshold approach seeks to enhance long-term yield of a population and to maintain population renewability. I evaluated threshold management strategies for selected herring and pollock stocks in Alaska. First, I examined stock-recruitment data from 19 major herring stocks worldwide to provide the basis for evaluating threshold management strategies. Seventy-three percent of these stocks exhibited statistically significant density-dependence. Most stocks have compensatory, dome-shaped stock-recruitment curves. Then, I simulated threshold management strategies for eastern Bering Sea (EBS) pollock and herring and Prince William Sound (PWS) herring using a single-species model. I further examined seven alternative threshold estimation methods. Cohort analysis, catch-at-age analysis, and catch and population sampling yielded estimates of population parameters. The objective function was a weighted function of increased average yield and decreased standard deviation of yield over a planning horizon. Compared to a non-threshold approach, threshold management strategies increase the long-term average yields, stabilize population abundances, shorten rebuilding times, and increase management flexibility. For a maximum yield criterion and Ricker stock-recruitment models, optimal fishing mortalities are slightly above fishing mortalities at maximum sustained yield (MSY), and optimal threshold levels range from 40% to 60% of pristine biomass for EBS pollock, from 40% to 50% for EBS herring and from 30% to 60% for PWS herring. With fishing mortality at MSY and the criterion of equal trade-off between yield and its variation, optimal thresholds range from 20% to 30% of pristine biomass for pollock. With the status quo exploitation rate of 20%, optimal thresholds range from 10% to 25% of pristine biomass for EBS herring, and from 5% to 25% for PWS herring. Of the threshold estimation methods evaluated, default percentage of pristine biomass usually performs best. Loss of yield due to errors in threshold estimation is small, generally under 10%. A bout 15 to 20 years of data are required to obtain a reliable estimate of thresholds. With single-species dynamics, the form of the stock-recruitment curve, exploitation rate and management objective are the most important factors affecting optimal thresholds

Trophic and life history considerations with respect to multispecies management policies

As fisheries become more heavily exploited, the need for multispecies management has become more apparent and trophic relationships are cited as the primary cause for this need for multispecies management. This thesis investigates one aspect of the impact of trophic relationships on multispecies management; indirect effects on food chains by predators. The approach chosen to investigate the impact of trophic relationships on multispecies harvesting is a simulation one. Although the results of a simulation approach are not as powerful as those from an experimental study, the practicality in terms of testing alternative management plans is much higher. The model is of the Catalina nearshore benthic fish community. The choice is a practical one. The feeding habitats of the Catalina nearshore benthic fishes have been studied extensively and a data base of this quality and breadth is necessary for the parameter estimation for this modeling effort. This thesis is divided into four parts: 1) the analysis of feeding selectivity, 2) a review of feeding of benthic fishes at Catalina Island, 3) the modeling effort itself, 4) a review of population parameters and life history patterns of marine fishes

Winter School on Towards Ecosystem Based Management of Marine Fisheries – Building Mass Balance Trophic and Simulation Models

Winter School on Towards Ecosystem Based Management of Marine Fisheries – Building Mass Balance Trophic and Simulation Model

Small scale multispecies demersal fishery off Negombo, Sri Lanka : a study of their biology and socio-economics

The present multidiciplinary study on the demersal fishery in the Negombo area on the west coast of Sri Lanka was based on the data collected from 1992 to 1999. The prime objective of this study was to identify an appropriate management strategy for the sustainable development of the resources and the fishery. Assessments of fishery, fishery resources, economics of fishing marketing and the social aspects of the fishing community were studied in detail. The status of economically important fish stocks in the shallow and deep waters were assessed through indicator species. Lelhhnus lentjan and Leihrinus nehulosus. The demersal fishery in the area is highly diverse and the highest fishing effort is deployed by handline combined with drift gillnet boats followed by bait cage traditional handline with outboard motor boats, bottom trammel net. bottom longline. bait cage handline with inboard motor boats, and a more limited effort by bottom set gillnet and spear fishing. Handline with inboard motor boats, bottom set gillnets and spear fishing only operate during the nonmonsoon season, but fishing effort is high during this period by all gears. The multigear demersal fishery in the area is predominately conducted in the shallow waters of less than 40 m and only handlinc and bottom longline fishing are deployed in depths greater than 40m. The CPUE realised from shallow waters are low for all gears but improved with increasing fishing depth. A total of 139 fish species belonging to 68 families have been recorded in the catches but the most important families arc Lcthrinidac. Carangidac. Lutjanidac. Serranidae and Scombridac /.. nehulosus and /,. tentjim arc the dominant species. Recently the contribution of squid and cuttlefish to the total demersal catch has increased. Lcthrinds replaced the catches of Carangids as dominant fish. The three important gears, traditional bait cage handlinc. bottom longlinc and bottom trammel net fisheries arc highly interactive, harvesting the same stocks of economically important species of different but overlapping sizes. Both traditional handlinc and bottom trammel nets catch large quantities of juveniles of the indicator species inhabiting the shallow waters while bottom longlincs catch adults in deeper waters. A decline of CPUE of these interactive gears has been observed over the years. The present fishing effort of the multispccics demersal fishery has come close to the optimum, which produces the ma.ximum sustainable yield (MSY). but has long c.xcccdcd the maximum economic yield (MEY). The economically important fish resources in the shallow waters arc being overexploited and have long c.xcccdcd the optimum exploitation of 0.5 by all three main gears. The exploitation of bottom longlinc fishing has exceeded the optimum effort which produces MSY for nehulosus and both handlinc and bottom longlinc has c.xcccdcd the optimum cfTort for A. lentjan. The economics of exploitation of these two species has c.xcccdcd the MEY by all gears. The yield or the value of the catch of these two indicator species could only be improved by a 50% reduction of current effort of cither bottom trammel nets or traditional handlinc fishery, but over 60% of the households engaged in these fisheries depend entirely on fishing income. All boat/gcar combinations involved in demersal fishing c.xhibitcd good performance and generated a positive net profit, but the economic performance among them is highly variable. Profitability is highest for modem gears rather than traditional bait cage handlinc fishing. The seasonal change in fishing, according to the seasonal abundance of resources, is economically rewarding. Relatively low fixed costs plus a competitive market, high demand and low indebitness to middlemen results in a high net profit

EcoJogical-economic modelling and implications of land use change and wetlands extent on freshwater fisheries: the case of Lake Victoria (East Africa)

Watershed degradation and the conversion of wetlands to alternative uses affect water quality, having profound implications for the freshwater ecosystem and fisheries of Lake Victoria. Through dynamic simulations using the Ecopath approach we show that eutrophication may explain the sudden upsurge of the introduced Nile perch during the 1980s. During these years an important trigger of change was an abrupt shift in primary productivity due to an external shock, probably related to an El Nino-ENSO event. The Nile perch population explosion during the 1980s caused a profound transformation ofthe fishing industry leading to a dramatic increase in fishing effort. Our equilibrium analysis from the base values of an Ecopath mo~l for year 1989 depicts an overfishing situation. To quantify the combined effect of eutrophication and fishing effort on the-aggregate fish stock and fishery we specify and estimate a biomass dynamics model, which embeds a measure of phytoplankton biomass as an environmental variable. We found that in a eutrophic state, which for the Kenyan waters of Lake Victoria was above an estimated chlorophyll-a concentration threshold of 17.11 mg m-3 , a further increase in phytoplankton biomass would cause a decrease in aggregate fish stock biomass and landings respectively. We then model the relation between land use change and water quality, considering the nutrient retention function of wetlands. First, we identify catchment-based indicators, in particular price indicators and wetland extent, which may anticipate changes in ,. environmental variables driving stock assessment models. We then study both nutrient loading and wetland reclamation externalities to the fishery sector, focusing on the Yala swamp, one of the largest wetlands in the Kenyan basin of Lake Victoria. Finally we propose a spatially distributed land use tax as policy instrument for reaching the desired objective of changing farmers' behaviour and maximising the total economic benefit to society

Interactions of the South African fur seal and fisheries in the Benguela ecosystem

Bibliography: pages 168-180.This thesis adopts an holistic approach to evaluation of the "seal problem", which encompasses the operational and biological interactions between the South African fur seal and fisheries in the Benguela Ecosystem. The operational interactions are examined using a qualitative assessment, and speculative, single-species simulation models are developed to improve understanding of seal population dynamics, present options for seal management and explore seal-fishery interactions. The models show that the probability of a seal living longer than 20 years is 14%; that the population has increased at a rate of 5.1 % p.a. since the last major census in 1983; that in 1988 there were an estimated 1.5 million seals that consumed over 2 million tons of fish; that although seal population size is usually estimated by multiplying the number of pups by a factor of 4, this factor is very variable and under different culling regimes is usually greater than 4; that historically, the mean population : pup ratio is 4.55. The most effective means of reducing the population is to cull both pups and bulls (the population decrease resulting mainly from reduced pup production caused by disturbance during bull culling). Culling pups is the least effective means of population control. Culling cows alters the population sex ratio drastically and is considered undesirable. To achieve a small or negative population growth rate, more bulls and pups need to be culled than have been taken historically. Each of the fisheries is found to be subject to operational interactions with seals, this being most severe for the purse-seine fishery. The per capita consumption of fish by seals is similar under any management regime so it is not justified to adopt a particular culling strategy to reduce overall consumption. When either seal predation or fishing mortality is reduced, the model simulates an increase in fishery yield of Cape hakes. But there is inadequate understanding of hake population dynamics at present, and depending on the interactions between species, decreased predation may result in increased or decreased fishery yields. This study shows that operational interactions are a real problem and this biases the opinion of fishermen towards seals as competitors for fish resources