Sampson, D.B. and Scott, R.D. 2012. An exploration of the shapes and stability of population-selection curves. Fish and Fisheries 13: 89-104.
Abstract
Fishing operations on any given stock rarely generate fishing mortality that is uniform across all ages and sizes. Population-selectivity refers to a scaled version of the age- or size-specific fishing mortality experienced by a fish population. Although it is common to apply a sigmoid logistic curve for the selectivity produced by many kinds of fishing gear, the general characteristics of population-selection curves have not been well examined. In this study, generalized additive models were fit to sets of selection coefficients taken from 15 recent stock assessments conducted using the virtual population analysis approach. The selection coefficients predicted by the models provided smoothed representations of the shapes and temporal dynamics of selectivity. Four broad types of selectivity were found: increasing, asymptotic, domed, and having a saddle. Four specific cases, each dominated by one type of selection curve, were examined in detail. For all 15 stocks, the population-selection curves were not stable through time but underwent changes in shape, which in some cases were quite radical. Temporal variation in population-selectivity has important implications for the conduct of fisheries modelling activities such as evaluating management strategies and forecasting catch and stock size.
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Sampson, D.B. 2011. The accuracy of self-reported fisheries data: Oregon trawl logbook fishing locations and retained catches. Fisheries Research 112: 59-76.
Abstract
Self-reporting of information is an inexpensive means for collecting data on fishing or other human activities, but the accuracy of self-reported data can be difficult to measure. Logbooks that record fishing activities are a common form of self-reporting that can provide vast amounts of information about a fishery. Using logbook data from trawlers from Oregon ports operating along the US Pacific coast during 1987 to 2001, this study examined (a) the consistency of reported tow locations and bottom depths and (b) the accuracy of reported retained catches of four groundfish species groups. Because logbook data during this period played no direct role in fishery management or stock assessments, there were no obvious incentives for trawl captains to deliberately report false logbook information. To evaluate consistency between tow locations and corresponding bottom depths, each tow was assigned to a 100-m bottom depth-band based on the reported tow location on a digital chart. The depth-bands based on the tow locations were then compared one-to-one with the depths reported in the logbooks and analyzed to determine factors (e.g., year, quarter, vessel) contributing to the consistency between the reported locations and depths. Consistency was highest in the shallowest depth-bands and there was a strong interaction between depth and vessel. To evaluate the accuracy of the retained catches reported by the captains, the tow-by-tow logbook data were summed to the fishing-trip level and compared to the official landing weights, recorded at the fish processing plants when the catches were off-loaded and weighed. The logbook weights relative to the corresponding official landing weights were analyzed using zero-inflated generalized Poisson (ZIGP) regression models to determine factors (year, quarter, vessel) that were important explanatory variables. The analysis found strong year, quarter, and vessel effects for all three parts of the ZIGP: the mean value, the dispersion, and the proportion of zeros. The vessel effect was strongest. In a combined analysis, based on results from the other two analyses, vessels that were consistent in recording their depths and fishing locations had only a weak tendency to be accurate in recording their catches.
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Sampson, D.B. and Scott, R.D. 2011. A spatial model for fishery age-selection at the population level. Canadian Journal of Fisheries and Aquatic Sciences. 68:1077-1086
Abstract
Different age classes do not generally experience the same rates of fishing mortality. The processes resulting in age- (or length-) selection operate at several scales. At the broadest scale, population-selection measures the age-specific probability of capture, while at the finest scale contact-selection describes the vulnerability of fish that encounter the fishing gear. Population-selectivity is the process most relevant to fish population dynamics and stock assessment, but it has received far less attention than processes operating at gear-specific scales. Despite wide recognition of the diverse shapes possible for population-selectivity, the processes determining these shapes are poorly understood. This paper develops a reasonably simple model of population-selectivity from a set of survival equations, coupled to allow movement between subpopulations, and explores the conditions necessary to produce different shaped population-selection curves. Important factors influencing the population-selectivity model are the gear-specific selection characteristics of the fleets, their effort levels relative to one another, the spatial distribution of fishing mortality, and the movement of fish between subpopulations. The model can generate quite complicated curves and has surprising properties. For example, under a wide variety of conditions, even though the same asymptotic gear-selectivity applies in all subpopulations, the overall population-selectivity will be dome-shaped unless fishing mortality is uniform across all subpopulations.
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Scott, R.D. and Sampson, D.B. 2011. The sensitivity of long-term yield targets to changes in fishery age-selectivity. Marine Policy 35:79-84.
Abstract
Maximum sustainable yield (MSY) has been adopted as the primary management goal by several inter-government fishery organisations, and in the United States it forms the cornerstone of federal fishery management policy. MSY became a strategic goal for the management of Europe´s fisheries following the resolution of the World Summit on Sustainable Development in 2002 to restore or maintain fish stocks to MSY levels by 2015. Calculation of MSY requires information on the rate at which biomass increases through growth and reproduction and the rate at which it decreases through natural mortality and fishing. Population-selection, which measures the age-specific rates of fishing mortality, is a key component for the calculation of yield as a function of fishing mortality and MSY, yet selection rarely features in either management advice or sensitivity analyses. Effective management of selection can potentially lead to increases in long-term yield, but before taking action managers need to understand what long-term increases are possible. Using a hypothetical stock, equilibrium yield curves were calculated for three scenarios in which the shape of the population-selection curve varied. The results illustrate the potential extent of variation in MSY and the corresponding fishing mortality required to achieve it (FMSY) that may result solely due to changes in population selectivity. They show that relatively subtle changes in selection can produce substantial differences in MSY and FMSY. The results are discussed with specific reference to the development of long-term management targets and the mechanisms by which managers might try to influence population-selection.
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Lee, Y.W. and Sampson, D.B. 2009. Dietary variations in three co-occurring rockfish species off the Pacific Northwest during anomalous oceanographic events in 1998 and 1999. Fishery Bulletin 107:510-522.
Abstract
Stomach samples from three rockf ish species, yellowtail (Sebastes flavidus), widow (S. entomelas), and canary (S. pinniger) rockfish, seasonally collected off the Pacific Northwest in 1998 and 1999, provided quantitative information on the food habits of these species during and after the 1997-98 El Niño event. Although euphausiids were the most common major prey of all three predators, gelatinous zooplankton and fishes were the most commonly consumed prey items during some seasonal quarters. The influence of the El Niño event was evident in the diets. Anomalous prey items, including the southern euphausiid species Nyctiphanes simplex and juveniles of Pacific whiting (Merluccius productus) frequently appeared in the diets in the spring and summer of 1998. The results of stomach contents analyses, based on 905 stomach samples from 49 trawl hauls during seven commercial fishing trips and from 56 stations during research surveys, were consistent with the timing of occurrence and the magnitude of change in biomass of some zooplankton species reported from zooplankton studies in the northern California Current during the 1997-98 El Niño. Our findings indicate that the observed variations of prey groups in some rockfish diets may be a function of prey variability related to climate and environment changes.
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Emmett, R.L. and Sampson, D.B. 2007. The relationships between predatory fish, forage fishes, and juvenile salmonid marine survival off the Columbia River: a simple trophic model analysis. Calif. Coop. Oceanic Fisheries Investigations Rep. 48:92-105.
Abstract
A trophic model that simulates interactions between a predatory fish (Pacific hake, Merluccius productus), forage fish, and juvenile salmon off the Columbia River was constructed to identify if trophic interactions could account for marine mortality of Columbia River juvenile salmon. The model estimates the number of juvenile salmon that are eaten annually by Pacific hake off the Columbia River for a given hake and forage fish population. Model results indicate that the presence of high numbers of Pacific hake could account for high mortality of some juvenile salmonid species/stocks leaving the Columbia River, and that this mortality would be much reduced when forage fish are abundant. Estimates of hake and forage fish abundance, based on field data collected from 1998-2005, were used in the model to derive annual estimates of the number of salmon possibly eaten by hake. A multiple regression analysis using the output from the trophic model and average May/June Columbia River flows accounted for much of the annual variation in Columbia River fall Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon marine survival (p < 0.05, R² > 60%), but not spring or summer Chinook salmon. For these two stocks, average May/June sea-surface temperature was the best predictor of marine survival. Results support the hypothesis that for some Columbia River salmon species/stocks, marine survival is predation-driven and affected by the interaction between the abundance of Pacific hake, forage fish, Columbia River flows, and possibly ocean turbidity. Future modeling work should include predation estimates of other large fishes, marine mammals, and sea birds.
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Lee, Y.W. and Sampson, D.B. 2005. Monte Carlo evaluation of a nonlinear regression estimator for aggregated length-weight data. Canadian Journal of Fisheries and Aquatic Sciences 62:362-373
Abstract
Parameters of fish length-weight relationships (W = a·L^b) are usually estimated by applying linear regression to log-transformed length and weight values, but measuring individual weights is time-consuming and expensive. Often length and weight data are available as sets of length measurements and aggregated sample weights, and the aggregate average weight of a sample can be expressed as the average of the weights predicted for the individual fish lengths. This study evaluated the feasibility of applying nonlinear regression to aggregated length-weight data. Experiments with simulated random length-weight data demonstrated that the estimates of parameter b appear to be unbiased and the estimates of a are right-skewed and biased. Further, the estimates of ln(a) and b are almost perfectly correlated. The precision and accuracy of the estimates were greatly influenced by the number of aggregate samples but were relatively unaffected by the number of fish in each sample. An additional experiment showed that the residuals from the regression can be used to detect small changes in the length-weight parameters.
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Yin, Y. and Sampson, D.B. 2004. Bias and precision of estimates from an age-structured stock assessment program in relation to stock and data characteristics. North American Journal of Fisheries Management 24:865-879.
Abstract
Assessments for many U.S. Pacific coast groundfish stocks have been developed using the statistical catch-at-age method known as Stock Synthesis. This study used Monte Carlo simulation and a fractional factorial experiment to evaluate the effects of input data errors and stock characteristics on bias and precision in estimates of ending exploitable biomass, rate of fishing mortality, depletion, and other output variables. Nine factors were examined: length of the data series, rate of natural mortality, shape of the fishery selectivity curve, trend in fishing mortality, recruitment pattern, and levels of sampling error in the data for catch, fishing effort, a survey biomass index, and fishery and survey age compositions. Length of the time series, age composition sample size, survey biomass variability, and fishing effort variability were the most influential factors for most of the output variables. The estimates of depletion had the least bias and the estimates of starting biomass the smallest variability; the estimates of ending recruitment the greatest bias and largest variability. For all the output variables examined the estimates appeared to be median-unbiased. For the conditions considered in the experiment it appears that accuracy of assessment estimates for ending exploitable biomass and projected catch would be more readily improved from increased age composition sampling than comparable (but much more expensive) improvements in survey estimates of stock biomass.
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Lee, Y.W. and Sampson, D.B. 2000. Spatial and Temporal Stability of Commercial Groundfish Assemblages off Oregon and Washington as Inferred from Oregon Trawl Logbooks. Canadian Journal of Fisheries and Aquatic Sciences 57:2443-2454.
Abstract
Spatial and temporal characteristics of species compositions for fifteen major commercial groundfish species off Oregon and Washington (USA) were studied using Oregon bottom trawl logbook data, 1987 to 1993. Screening procedures to remove questionable data from the original logbook files resulted in the exclusion of information from 46% of the total available tows. Two multivariate methods, detrended correspondence analysis (DCA) and Ward’s hierarchical cluster analysis were used to derive species ordinations and groupings. A general linear model developed for the primary DCA axis scores showed that the species compositions were strongly correlated with depth and minimally correlated with the other environmental variables examined (latitude, month, and year). However, there were distinct seasonal changes in species composition at the shallower depths, consistent with the movement of deepwater species onto the shelf during summer. Annual trends in the species compositions were not evident even though the study region is very dynamic oceanographically and was heavily fished during the study period.
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Helu, S.L, Sampson, D.B., and Yin, Y. 2000. The application of statistical model selection criteria to the Stock Synthesis assessment program. Canadian Journal of Fisheries and Aquatic Sciences 57:1784-1793.
Abstract
Statistical modeling involves building sufficiently complex models to represent the system being investigated. Overly complex models lead to imprecise parameter estimates, increase the subjective role of the modeler, and can distort the perceived characteristics of the system under investigation. One approach for controlling the tendency to increased complexity and subjectivity is to use model selection criteria that account for these factors. The effectiveness of several selection criteria was tested in an application with the stock assessment program known as Stock Synthesis. This program, which is often used on the U.S. west coast to assess the status of exploited marine fish stocks, can handle multiple data sets and mimic highly complex population dynamics. The Akaike Information Criterion (AIC) and Schwarz’s Bayesian Information Criterion (BIC) are criteria that satisfy the fundamental principles of model selection: goodness-of-fit, parsimony, and objectivity. Their ability to select the correct model form and produce accurate estimates was evaluated in Monte Carlo experiments with the Stock Synthesis program. In general AIC and BIC had similar good performance in selecting the correct model and they produced comparable levels of accuracy in their estimates of ending stock biomass.
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Sampson, D.B. and Al-Jufaily, S.M. 1999. Geographic variation in the maturity and growth schedules of English sole along the U.S. west coast. Journal of Fish Biology 54:1-17.
Abstract
Logistic regression analyses showed that the maturity versus length relationship for female English sole Pleuronectes vetulus varied significantly among sampling locations from California to British Columbia in June-September. Stepwise analyses found that latitude, bottom gradient, bottom temperature and sediment type were all significant factors. Regression analyses of age at length from subsamples indicated that latitude, maturity and bottom sediment had significant effects on growth. Data from the aged subsamples showed significant differences among age classes in the length versus maturity relationship with older fish attaining maturity at smaller sizes than younger fish. On a coastwide basis the length and age at 50% maturity were c. 230 mm and 4.7 yr respectively. Growth rate and length at 50% maturity have substantially decreased relative to observations from the 1950s, but age at 50% maturity has increased.
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Sampson, D.B. and Yin, Y. 1998. A Monte Carlo evaluation of the Stock Synthesis assessment program. In: Funk, F. et al. (eds) Fishery Stock Assessment Models. Alaska Sea Grant College Program Rep. No. AK-SG-98-01, University of Alaska Fairbanks, p.315-338.
Abstract
Stock assessments for many U.S. Pacific coast groundfish stocks are developed using the catch at age method known as Stock Synthesis. The Stock Synthesis computer program attempts to reconstruct the demographic history of a stock from observed changes in fish age or size distributions, coupled with auxiliary information such as an index of stock biomass developed from a research survey or an index of fishing mortality based on fishing effort. In this study Monte Carlo simulation techniques were used to generate fishery and survey data with known characteristics. The simulated data were then analyzed with the age-structured version of the Stock Synthesis program and results from the program were compared with the true values to evaluate the influence of measurement errors on the accuracy of the Stock Synthesis results. Data sets were constructed with low and high levels of random error in each of four types of sample data (fishery age composition, a fishing effort index, survey age composition, and a survey index of stock biomass). A series of experiments, based on a fractional factorial design, was conducted to examine the importance of eight factors: low versus high rates of natural mortality; constant versus variable annual recruitment; low versus high rates of increase in fishing mortality; dome-shaped versus asymptotic fishery selectivity; short versus long data series; low versus high variability in the fishing effort index; low versus high variability in the survey biomass index; and small versus large samples for age composition. On average the Synthesis estimates for total biomass, exploitable biomass, recruitment, and fishing mortality in the ending year were slightly positively biased (3.5-6.1%) but less variable than the input data. In general, the number of years in the data series and the size of the age samples were the most influential factors, with increased amounts of data producing less biased and less variable estimates.
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Sampson, D., Barss, W., Saelens, M., and Wood, C. 1997. The by-catch of sablefish, Anoplopoma fimbria, in the Oregon whiting fishery. Proceedings of the International Symposium on the Biology and Management of Sablefish. NOAA Technical Report NMFS 130:183-194.
Abstract
The fishery for Pacific whiting (Merluccius productus) has removed tremendous quantities of fish since the onset of the foreign fishery in 1966. Although this trawl fishery targets on midwater schools of whiting, other species are caught incidentally. This paper analyzes in detail the bycatches of sablefish (Anoplopoma fimbria) that were landed in 1992 by trawlers delivering whiting to onshore processing facilities in Oregon. During the period from May through October, port agents examined 49% of all deliveries, and recorded the bycatches of sablefish and other regulated species. About half of these deliveries included small sablefish, which ranged in length from 33 to 54cm with an average weight of about 0.9 kg per fish. The distribution of sablefish landed per delivery was highly over-dispersed, with an average of 24.6 fish per delivery and a standard deviation of 106.9. In addition to the shore-side sampling, 15% of the whiting trips from Oregon ports also had agency observers on board, who kept track of the catches from the time they were brought on board through to the off-loading at the processing plants. About half of the whiting trips caught at least one sablefish, and on about half of the trips that caught sablefish, some sablefish were discarded at sea. When discarding occurred, about one quarter of the sablefish were discarded. For the entire season we estimate that the shore-side fishery in Oregon caught about 31,400 sablefish, which represents about 0.35% of the estimated number of sablefish that recruit annually.
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Sampson, D.B. 1997. Predictability of groundfish catch rates and species mix in the United States west coast trawl fishery. In: Fisheries Bycatch: Consequences and Management. Alaska Sea Grant College Program Report No. 97-02. University of Alaska Fairbanks, p.45-46.
Extended Abstract
Fishing with a bottom trawl often results in the incidental capture of non-target fish species, but some fishing practices and locations generate greater amounts of by-catch than others. Identifying those fishing strategies that result in by-catch is an important first step to reducing the problem. If the distribution and composition of fish assemblages are transitory or unpredictable, then by-catch may be unavoidable without a complete ban on fishing.
In this study logbook data from the groundfish trawl fishery off Oregon and Washington were examined to assess the variability of catch rates and species composition. The data, which include skippers’ tow-by-tow estimates of retained catch, were compared with landing receipts to remove inaccurate information; trips influenced by regulatory trip limits were also excluded. A subset of the remaining data was chosen for detailed analysis to identify influential factors. Excluded from the analysis were boats that operated in a limited number of areas, and areas operated in by a limited number of boats.
The selected data were analyzed using generalized linear models of catch rates and species mix to measure the importance of the following factors: Year (1987-1993), Time-of-year (bimonthly intervals), Boat (25 vessels), Net type (generic bottom trawl, sole net, trawl with roller gear), and Area (30 regions defined by 20 minute intervals of latitude and 40 fathom intervals of depth). Because there were large numbers of tows with catches that were zero, catch rates were modeled using a delta-lognormal distribution; the numbers of tows with zero catch were treated as binomial random variables and the catch rates for the non-zero tows were treated as lognormal random variables.
The process of data verification and screening resulted in the exclusion of data from one third to one half of the fishing trips. The data subset examined with the generalized linear models consisted of tow-by-tow catch rates (lb/hr) from 15,341 tows for fifteen species or species groups.
In a logistic regression analysis of the zero-catch tows essentially all factors (Year, Time-of-year, Boat, Net, and Area) were highly significant (P<1%) for all 15 species, and Boat was the single most important explanatory variable for 11 species. For 13 species the Boat.Year interaction was the single most important pairwise interaction. The results were less uniform across species in an analysis of variance of the logarithm of the non-zero catch rates. Essentially all factors again were highly significant for all 15 species, but Boat was the single most important explanatory factor for only eight species (including all six of the rockfish species). In a logistic regression analysis of species co-occurrence in individual tows the factor Boat was the single most important explanatory variable for many of the species combinations.
The results of the analyses generally indicate that catch rates and species mix can be fairly well predicted by relatively simple statistical models, but for most species there are highly significant boat to boat differences. The fact that boats produce different rates of species co-occurrence suggests that some fishers are better able to avoid bycatch than others. From a detailed investigation of the fishing practices of individual fishers it might be possible to develop improved methods for bycatch avoidance.
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Lawson, P.W. and Sampson, D.B. 1996. Gear related mortality in selective fisheries for ocean salmon. North American Journal of Fisheries Management 16:512-520.
Abstract
In ocean fisheries for salmon there can be several forms of gear related mortality. Much research effort has been directed at estimating mortality rates for salmon that are hooked and then released. Also potentially important, but not easily measured, is mortality of fish that escape from the hook before being brought to the boat or fish that are removed from the hook by predators, so-called “drop offs”. In selective fisheries, where some hatchery bred fish are marked for retention and unmarked fish legally must be released, the actual mortality rate suffered by unmarked fish depends on the harvest rate for the marked fish, the accuracy of mark recognition, and the proportion of marked and unmarked fish at the beginning of fishing. This paper develops a model for evaluating gear related mortality in selective fisheries and explores the potential importance of several sources of mortality. Mortality rates for unmarked fish are generally lower than the apparent harvest rates, but increase rapidly as harvest rates increase. With respect to the overall mortality of unmarked fish, drop off mortality could be as important as hook and release mortality.
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Sampson, D.B. 1994. Spatial patterns associated with an increase in the abundance of English sole (Parophrys vetulus) off Oregon and Washington. In: Proceedings of the International Symposium on North Pacific Flatfish. Alaska Sea Grant College Program Report No. 95-04, University of Alaska Fairbanks, p.367-384.
Abstract
A recent assessment for the stock of English sole (Parophrys vetulus) off Oregon and Washington (USA) indicates a large and steady increase in the numerical abundance, biomass, and annual recruitment of females during the period 1977-92. Abundance and biomass levels in 1992 were approximately ten times the 1977 levels. This paper analyzes the time series of research trawl data to examine how the spatial distribution of this resource changed as the stock increased. There appears to have been an increase in the density of the fish but relatively little expansion of their range. The results are discussed in relation to MacCall’s basin model for the dynamic geography of marine fish populations.
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Sampson, D.B. 1994. Fishing tactics in a two species fisheries model: the bioeconomics of bycatch and discarding. Canadian Journal of Fisheries and Aquatic Sciences 51:2688-2694.
Abstract
The selections for fishing location largely determine the species mix and value of a fisher’s catch. Because of travel costs, these choices also determine the profitability of a fishing trip. This paper develops a simple theoretical model for the selection of fishing locations by a fisher faced with two co-occurring species whose densities vary with distance from port. For each species there can be different catchability coefficients, handling times, and prices. The duration of each fishing trip is assumed fixed. The model is used to determine the profit-maximizing fishing tactics (the fishing locations and time spent at each location), and to explore the conditions that generate deliberate bycatch and discarding when one of the species cannot be sold because it is unmarketable or because of trip limits.
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Sampson, D.B. 1994. Estimating the number of fish landed from their total weight and a sample average weight. Canadian Journal of Fisheries and Aquatic Sciences 51:2537-2548.
Abstract
Assessing the status of a commercially exploited fish stock usually involves fitting a model for the stock’s dynamics to observations of the number of fish landed. Typically the number of fish landed is estimated from the ratio of the total landed weight, which is known exactly, over the sample average weight of a fish. This paper examines how errors in the sample average weight are transformed asymmetrically into errors in the estimate of the numbers landed. Two alternative estimators are developed and are shown, using Monte Carlo simulations, to be less biased than the standard estimator.
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Sampson, D.B. 1994. A bioeconomic model for fish harvest technology. In: Proceedings of the Sixth Conference of the International Institute of Fisheries Economics and Trade, Vol. I, p.197-207. Ed. by M. Antona, J. Catazano, and J. Sutinen. IFREMER, Issy-les-Moulineaux, France.
Abstract
The catches and profits in a fishery depend on the technology employed, on the utilization of human and capital resources, on the abundance and distribution of the fish stock, and on the prices for fish, fuel, and labor. Here a harvest function is constructed that includes parameters for the main technical components of the fishing process: travel to the fishing grounds, catching fish, and handling those that have been caught. The fishery system does not have the same sensitivity to each technical parameter. Furthermore, small changes in a parameter can result in catastrophic alteration of the system’s dynamics. Fishing technology in part determines the behavior of the fishery system; the technology in turn evolves in response to changes in the fishery. It is argued that fishing technology will tend to evolve more rapidly in those components that have the greatest impact on the flow of fishing profits.
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Sampson, D.B. 1993. The assumption of constant selectivity and the stock assessment for widow rockfish. Fishery Bulletin 91:676-689.
Abstract
Modern methods for fish stock assessment are often based on age-structured models that separate each coefficient of fishing mortality at age into a time-specific factor (the rate of fishing mortality on the fully exploited age-classes) and an age-specific factor (a selectivity coefficient that measures the relative vulnerability of the particular age-class). The assumption that the selectivity coefficients are constant through time greatly simplifies the assessment process because it allows for a reduction in the number of unknown parameters. However, if the assumption is incorrect, it can lead to incorrect estimates of the stock’s status.
The most recent assessment for the stock of Pacific widow rockfish (Sebastes entomelas) is based on the untested assumption that the selectivity coefficients have not changed over the years. This assessment is derived from an analysis of catch-at-age data using an assessment method known as the Stock Synthesis program. The work described here examined the sensitivity of the assessment to the assumption of constant selectivity. Simulation experiments with the Stock Synthesis program showed that the assessment for widow rockfish can be highly sensitive to modest changes in selectivity. Experiments with two other assessment techniques, which also assume constant selectivity (the CAGEAN program of Deriso, Quinn, and Neal and the multiplicative catch-at-age model of Shepherd and Nicholson), showed that these methods are similarly sensitive to changes in selectivity.
