Michael Pennington

Assessing the Effect of Intra-Haul Correlation and Variable Density on Estimates of Population Characteristics from Marine Surveys

In a previous paper (Pennington and V0lstad, 1991, Biometrics 47, 717-723), it was suggested that reducing the size of the sampling unit currently used in marine surveys could increase the precision of the resulting density estimates. But if unit size is reduced, fewer animals will be caught during a survey. Concern has been expressed that this reduction in total catch would lower the precision of estimates of population characteristics, such as age and length frequency distributions, of importance for stock management. In this paper we examine the effect of sampling unit size, intra-cluster correlation, and variable density on the precision of estimates of population characteristics. An examination of some survey data indicates that reducing the size of the sampling unit employed and using the time saved to take samples at more locations could also yield more precise estimates of population parameters.

Effect of tow duration on length composition of trawl catches

Abstract The standard tow duration for trawl surveys is normally from 30 min to 2 h. In this paper we investigate if catch rates for different size groups change for varying durations of from 5 min to 2 h. Data for cod, haddock and long rough dab from the Barents Sea and Georges Bank are analyzed. Owing to small fish-large fish differences in swimming capacity, a relative decrease in catch rates of large fish with decreasing tow duration is expected. The results show no significant change in mean length with varying tow duration. It is indicated that short tows are at least as efficient as long tows in catching fish of any size. A hypothesis is proposed to explain this discrepancy from expectation. The results suggest that the efficiency of trawl surveys can be increased by reducing tow duration.

Surveys as a research tool for managing dynamic stocks

It is often assumed that assessments based on commercial catch and effort data, such as a virtual population analysis, will track trends in abundance more accurately than assessments produced by research surveys. Catch and effort data from a fishery provide information on the fish that were caught. For many fisheries, assessments based on catch data, even those incorporating survey data, are quite variable and may give a misleading picture of the health of the entire stock. Though abundance estimates generated by marine surveys are also variable, they often appear to track abundance trends more accurately than techniques that use catch data. Some statistical methods for analyzing survey data are briefly reviewed and comparative case studies of the assessments of the Newfoundland northern cod stock, the Namibian hake stocks, and Barents Sea cod and haddock are presented. Given the uncertainties inherent in a catch based assessment, it is concluded that surveys should be used to generate an assessment independent of catch statistics as a check of the catch based estimates.

An evaluation of recruitment indices for Arcto-Norwegian cod (Gadus morhua L.)

Abstract Abundance indices for Arcto-Norwegian cod (Gadus morhua L.) at various life stages were analysed to determine the index that provides the earliest reliable prediction of year-class strength. The indices considered are an egg abundance index; an early juvenile (approximately 3-months old) abundance index; 0-group (age 4- to 5-months) abundance indices; bottom trawl and acoustic survey abundance indices for 1-, 2- and 3-year-old cod, and VPA estimates of the abundance of 3-year-old cod and of spawning stock biomass. Based on a regression analysis, a cohort’s relative abundance as early juveniles is the best early indication of its abundance as 2- and 3-year-olds.

Cost-efficient survey designs for estimating food consumption by fish

Abstract This study examines the effect of survey design on the precision of estimates of average weight of stomach contents of fish. Since prey distribution is often patchy, fish at a station tend to have more similar stomach contents than do fish from different stations. Theory for evaluating the effects of intra-cluster correlation and variable density on the variance of estimates of average stomach contents is developed, and the implications for stomach sampling programs are examined. It is demonstrated that because of intra-cluster correlation the number of stations, more so than number of stomachs collected, determines the precision of estimated average stomach contents. As an example, the survey design for estimating the average amount of capelin ( Mallotus villosus ) in Barents Sea cod ( Gadus morhua ) stomachs in the winter is assessed, using survey data from years with high and low capelin abundance. The results, in agreement with the theoretical findings, show that precision would be maximized for fixed cost if fewer fish were collected from each station, and the freed resources were used to collect fish from as many stations as possible. In 1992, the survey design was changed accordingly; stomachs are now collected from twice as many stations, but the total number of stomachs collected has been reduced. Sampling fewer stomachs at each station enables most stomachs to be analyzed at sea.

Measuring the effect of changes in catchability on the variance of marine survey abundance indices

Abstract The average catch per tow by a research vessel survey is often used as an index of abundance. An estimate of the variance of such indices that is based only on the between station variability in catch may underestimate the true variance if catchability varies over time. In this paper, the survey index variance is estimated indirectly by cross-calibrating time series of virtual population analysis estimates and trawl survey indices of abundance. The method is applied to surveys of some fish stocks on Georges Bank, in southern New England, and in the Barents Sea. For these surveys, it appears that the true variance of the survey indices is approximately twice as large as the usual estimates based on the within survey variance. As an application, a time series technique, which requires an estimate of the survey index variance, is used to generate a more precise index of abundance. The results indicate that for the surveys examined the variance of the estimated abundance index is 30–40% smaller than the original (average catch per tow) index.

Variability of tubule types within the digestive glands of Mercenaria mercenaria (L.), Ostrea edulis L., and Mytilus edulis L.☆

Abstract The variability of the four tubule types previously recognized in the digestive glands of bivalve molluscs (viz. I, holding; II, absorptive; III, fragmenting; and IV, reconstituting) was investigated using photomicrographs of the digestive glands of subtidal Mercenaria mercenaria (L.), low-intertidal Ostrea edulis L., and mid-intertidal Mytilus edulis L. The clustering of similar tubule types around common secondary ducts was observed both histologically and by statistical analysis. A ratio type estimator was used in consideration of this clustering to determine proportions and variances of each tubule type. Intra-animal variances of Type II tubules were approximately the same for each species, ranging from 0.0419–0.0570. Inter-animal variances were also similar (0.0060–0.0140), but only 10–25% of the intraanimal values. Sampling schemes involving large numbers of animals but few photomicrographs of each digestive gland would minimize overall variance. Because of high variability, the necessity of taking numerical data and of using proper statistical analysis in all studies based on the changing morphological appearance of the bivalve digestive gland is stressed.