Ellen K. Pikitch

A Bayesian estimation and decision analysis for an age-structured model using biomass survey data

Abstract The use of Bayesian estimation and decision analysis is described and applied to estimate the probability distribution of virgin stock size, and expected consequences of future quotas for an age- and sex-structured model fit to biomass survey data. It is shown that Bayesian methods can be easily used with complex models when uncertainty in a single parameter is considered. An example from the fishery for hoki (Macruronus novaezelandiae) in New Zealand is presented, in which the weight, vulnerability schedule, fecundity schedules, natural mortality rate and the sensitivity of recruitment to spawning stock are assumed known. Results with a uniform prior probability on stock size are compared with those where an informed prior distribution is available. Three alternative harvest policies are compared and the expected values of several indicators of performance are computed for different quotas. The problems in extending the model to calculate Bayesian posterior distributions with multiple indicators of abundance, many more free parameters, and a stochastic model are considered.

Reproduction of Darkblotched Rockfish off the Oregon Coast

Abstract Seasonal reproductive development, size and age at sexual maturity, and fecundity were described for darkblotched rockfish Sebastes crameri collected off the Oregon coast. Altogether, 1,060 fish captured by commercial groundfish and shrimp trawlers between July 1986 and July 1987 were examined. Reproductive events were protracted. Insemination of females occurred from August to December, and fertilization and parturition followed from December through March. Spermatozoa were observed within both vitellogenic and previtellogenic ovaries between July and November. Reproductive events in smaller males and females were delayed relative to those in larger individuals. Females attained 50% maturity at a greater size (36.5 cm total length) and age (8.4 years) than males (29.6 cm total length; 5A years). The unimodal development of eggs and larvae indicated one parturition per year. Most age-6, age-7, and age-8 females possessed ovaries in an intermediate “maturing” condition. Histological analysis revea...

Animal population dynamics: Identification of critical components

Abstract There is a growing interest in the use of population dynamics models in environmental risk assessment and the promulgation of environmental regulatory policies. Unfortunately, because of species and areal differences in the physical and biotic influences on population dynamics, such models must almost inevitably be both complex and species- or site-specific. Given the emormous variety of species and sites of potential concern, this fact presents a problem; it simply is not possible to construct models for all species and circumstances. Therefore, it is useful, before building predictive population models, to discover what input parameters are of critical importance to the desired output. This information should enable the construction of simpler and more generalizable models. As a first step, it is useful to consider population models as composed to two, partly separable classes, one comprising the purely mechanical descriptors of dynamics from given demographic parameter values, and the other describing the modulation of the demographic parameters by environmental factors (changes in physical environment, species interactions, pathogens, xenobiotic chemicals). This division permits sensitivity analyses to be run on the first of these classes, providing guidance for subsequent model simplification. We here apply such a sensitivity analysis to network models of mammalian and avian population dynamics.

A histological description of shortspine thornyhead,Sebastolobus alascanus, ovaries: structures associated with the production of gelatinous egg masses

SynopsisThe ovarian structure of the shortspine thornyhead,Sebastolobus alascanus (Scorpaeniformes), which is similar to the ovarian structure previously described only for the pigmy lion fish,Dendrochirus brachypterus (Scorpaeniformes), is specialized for the production and expulsion of pelagic gelatinous egg masses. The germinative tissue and oocytes ofS. alascanus encircle a mass of spongy stroma that is located within the center of the ovary. The stroma is attached to the ovary wall only at the anterior end of each ovarian lobe; hence, the ovarian lumen surrounds the stroma, germinative tissue, and oocytes. Secondary oocyte development takes place on the ends of vascularized peduncles that are protrusions of the ovarian stroma. A gelatinous material is simultaneously secreted into the ovarian lumen by a single row of specialized cells that line the ovary wall. Eggs detach from peduncles and ovulate into the gelatinous material.

Effects of catch size and codend type on the escapement of walleye pollock (Theragra chalcogramma) from pelagic trawls

Abstract The effectiveness of codend meshes in permitting the escapement of undersized walleye pollock ( Theragra chalcogramma ) in the Bering Sea pelagic trawl fishery was tested during 13 July–1 August 1994. The alternate-haul method was used to compare catches made with treatment codends (i.e. large mesh codends) and a standard codend (i.e. small mesh codend). Four catcher vessels towed test codends that were fitted directly to their trawl gear. Codends were detachable, and catches were delivered to a factory trawler where sampling took place. Catch weights using treatment codends ranged from 0.3 to 79 t. Escapement of undersized pollock (less than 36cm) decreased as catch size increased for all treatment codends, until the retention of small pollock in the treatment codends was not significantly different from that in the standard codend (i.e. when catches of treatment codends exceeded 40 t). Potential benefits of codend mesh size regulations for this large-volume fishery are therefore debatable unless catch sizes are somehow restricted.

The statistical design of comparative fishing experiments

Abstract Experimental designs for alternate tow gear experiments are described and evaluated, considering situations with and without randomized complete blocks. Variance components relevant to the performance of these designs are identified, and a method for estimating them is presented and applied using data for the Pacific multispecies groundfishery. The aim of the Pacific experiment was to estimate the magnitude of changes in fish length for various species and total tow catch value due to changes in cod end mesh size. Alternative experimental designs are compared with regard to the sample sizes needed to reject the null hypothesis that there is no treatment effect. For the specific application examined, there is a 4–10-fold reduction in sample size when randomized complete blocks are used within a vessel trip, compared with an unblocked experimental design using only one treatment type per vessel trip.

Incidental Catch of Chinook Salmon in Commercial Bottom Trawls off the U.S. West Coast

Abstract Data on chinook salmon Oncorhynchus tshawytscha caught during commercial bottom trawl operations off the U.S. west coast were collected during 1985–1987 and 1988–1990. Almost no salmon were encountered in depths exceeding 482 m. Chinook salmon were caught most frequently and in greatest quantity during winter months, when catches were dispersed throughout a depth range of 100–482 m. Catches were relatively low during summer and occurred primarily in shallow water (at depths less than 220 m). Chinook salmon catch rates were higher off the south-central Oregon coast (Pacific States Marine Fisheries Commission [PSMFC] area 2B) than in more northerly coastal areas sampled during the 1985–1987 study. The 1988–1990 study, however, indicated higher catch rates off northern California (PSMFC area IC), south-central Oregon (area 2B), and northern Washington (areas 3B–C) than in other coastal areas. Chinook salmon catch estimates by bottom groundfish trawls during 1987 totaled 7,761 fish for areas 2B, 2C, ...

The predictive power of empirical relationships describing size selectivity, with application to gadoid fish

Abstract The use of empirical linear relationships between 50% retention length ( L 50% ) and codend mesh size ( MS ) of historical selectivity experiments has been suggested whenever selection data on a particular fish species are not readily available. However, when this procedure has been used in prediction, the predictive power of the fitted regression lines has been largely disregarded. Regression lines with large coefficients of determination (r 2 ) usually significant at α=0.05 or α=0.10, are considered appropriate, and the estimated regression coefficients are used to predict L 50% values for given mesh sizes without any consideration for prediction errors. We analyzed historical data of 689 selectivity experiments for 12 gadoid fish species and calculated an all-gadoids regression line as L 50% =−0.0831 + 0.344 MS ( r 2 =0.739). The predictive power of the regression was assessed by means of the ‘one item out’ cross-validation technique, and was found to be low in spite of the relatively high coefficient of determination. We investigated the effects of more homogeneous data sets on r 2 and predictive power by repeating the regression and cross-validation analyses on subsets including the results of selectivity experiments carried out on the same species, with the same experimental method (alternate-haul or covered codend) and codends of similar material. We found that (1) many simple linear models generally have low predictive powers in spite of their relatively large coefficients of determination, which reduces the value of their predictions for assessment or management purposes, and (2) linear regressions based on more homogeneous data subsets often display smaller prediction errors and larger coefficients of determination. Consequently, we suggest that the predictive power of empirical relationships describing size selectivity should be considered to select the regression line that provides more precise estimates of L 50% .

Stock Delineation of Dover Sole in the California–British Columbia Region, Based on Tagging Studies Conducted during 1948–1979

Abstract During 1948–1979, 34,377 Dover sole Microstomus pacificus were tagged and released in the California–British Columbia region, and 4,275 (12.4%) were subsequently recovered. A coastwide, interagency investigation began in 1985 to analyze the results of these studies with respect to stock delineation. Results of the tagging studies, and of other studies involving age determination, established that the Dover sole is a long-lived species. Maximum time at liberty for recoveries ranged from 13 to 22 years among studies, although 89–97% of the recoveries came within the first 5 years after tagging. Age-determination studies suggested a maximum age in the range of 36–52 years. Thus, there was adequate time for the tagged fish to disperse from their respective tagging areas. Few tagged Dover sole were recovered in international statistical areas north or south of their tagging area. Recoveries indicated that the fish occupied shallow water ( 199 m) during autu...