Jon Helge Vølstad

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.

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.