Olav Rune Godø

Growth and maturation of Norwegian coastal cod and Northeast Arctic cod under different conditions

Abstract The Norwegian coastal cod and the Northeast Arctic cod have been considered as two populations which can be separated by otolith studies. They are, in addition, characterized by differences in growth, age at first spawning and migration pattern. The environmental conditions under which the two groups live before maturation differ considerably. In this paper, growth and age at first spawning of cod from both groups, as well as of progeny from crossings between them, were studied under controlled laboratory conditions. The results show that growth increased and age at first spawning decreased considerably under favourable laboratory conditions compared to observations in the field. It is further indicated that the differences in growth and age at first spawning for coastal and Northeast Arctic cod reflect differences in the environment rather than genetic variations.

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.

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.

Size selection during trawl sampling of cod and haddock and its effect on abundance indices at age

Abstract Investigations of length selectivity of the standard Norwegian bottom trawl has revealed that small cod and haddock were largely under-represented in the catch. To minimize the effect of length selection on abundance indices at age obtained from surveys, a new standard sampling trawl has been used since January 1989. Based on parallel trawling experiments, conversion factors for comparing catches from the two sampling gears are available from previous work. Using these factors, data from 1983 to 1988 are adjusted to produce indices consistent with those from 1989 and 1990 which were obtained with the new sampling trawl. The new time series appears to be more accurate than the old series, which was biased by large yearly variability in the growth rates of small fish. Cod were more affected than haddock. The validity of combining results from two different sampling trawls into one index time series is discussed.

The use of tagging studies to determine the optimal time for estimating acoustic abundance of spawning cod

Abstract Abundance estimates of the spawning stock of northeast Arctic cod have, in recent years, been made on the basis of acoustic surveys. Migration in and out of the surveyed areas during investigation has been considered as a possible major source of error in the estimates. This paper evaluates the possibility of using recapture results from tagging experiments to illustrate migration to and from the Lofoten spawning area. Further optimal time period and survey strategy for such surveys are discussed. Tagging-recapture results seem to illustrate the movements of cod around the time of peak spawning reasonably well. Year-to-year variations are demonstrated. It is concluded that the effect of migration on the acoustic estimates of spawning cod in Lofoten is minimized by carrying out the survey during the last 3 weeks of March.