Immigration and emigration of cod in the spawning areas of Lofoten and Møre

Abundance estimates of the spawning stock of north east Arctic cod have in recent years been given on the basis of acoustic surveys. Migration in and out of the surveyed areas during investigation has been considered as a possibly major source of error in the estimates. By studying the migration of tagged cod in the spawning grounds of Lofoten and Møre between 1975 and 1981, it was found that the main immigration to these spawning grounds is compleeted by March 20 and that considerable emigration starts around April 10. It is thus concluded that the most facourable time to estimate the spawning stock, when taking into acount migration alone, is between these dates

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

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 VPA estimates and trawl survey indices of abundance. The method is applied to several species on Georges Bank and to haddock 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

Changes in acoustic abundance - a signal indicatior for stock dynamics for cod (Gadus morhua)

To be able to take the proper and timely management actions, it is crucial to reliably defect major changes in trends of fish stock abundance. For the assessment of Northeast Arctic cod, the Norwegian bottom trawl survey is important for tuning the VPA (Virtual Population Analysis). The indices are auto-correlated and the time series show maxima with vaguely defined peaks. This might lead to a delay in detection of the first signs of stock reduction. Norway has conducted combined acoustic and bottom trawl surveys for the main demersal fish stocks in the Barents Sea since 1982. The idea of combining the results from the two surveys into one estimate of abundance has been explored, but a number of confounding factors related to natural and affected fish behaviour has until1 now prevented the establishment of a reliable method for combining the two indices. The acoustic indices receive less weight or power during the tuning process than those based on the bottom trawl survey. There are, however, specific differences in the dynamics of the indices of the two time series that have not yet been exploited. In this paper these differences are analysed with the aim of extracting patterns that can serve a.s a. signal for changes in trends'of stock development. Application of the results for monitoring the stock is discussed

The spawning season fishery of cod at the Møre - Sør-Trøndelag coast in 1980

In 1980 most of the fishing effort was concentrated in the Breisund and on the Buagrunn. Gill net was the main fishing gear. Both coastal cod and mature Arctic cod (skrei) were caught. From February to April a sampling programme was carried out. Data on age, length, sex, maturity and stomach content were sampled. The year classes 1970-1972 dominated in the skrei catches, but throughout the season there seemed to be considerably more of the 1970 and 1971 year classes in the Breisund than on the Buagrunn. The proportion of younger skrei (the 1972 and 1973 year classes) increased from February to March on both fishing grounds. The older skrei reach the spawning stage earlier in the year than the younger. Therefore, the age distributions might indicate that while the younger skrei were feeding on rich herring concentrations at the Buagrunn during maturation, the older were spawning in the Breisund. The age composition in the catches from the coast of Sør-Trøndelag were similar to the Breisund ones. The age distribution in the gill net catches from Lofoten were dominated by 7 and 8 year old skrei, while the Breisund catches mainly consisted of 9 and 10 year olds. An estimate of the age composition of the mature stock based on VPA and a maturation curve give an intermediate age distribution. Tagging experiments indicate very limited exchange of spawning ground between skrei tagged in Lofoten and on Møre. The most likely reason of the observed variation of the age distributions in the two areas are suggested to be a combination of the following: -the part of the Arctic cod stock having the highest age of maturation recruits to the Møre fishery. -the recruitment to the two spawning areas is from different nursery and feeding areas. The coastal cod in catches with no skrei added, was dominated by 3 and 4 year old fish. In catches with excess of skrei, the coastal cod was dominated by fish older than 4 years

Immigration and emigration of cod in the spawning areas of Lofoten and Møre

Abundance estimates of the spawning stock of north east Arctic cod have in recent years been given on the basis of acoustic surveys. Migration in and out of the surveyed areas during investigation has been considered as a possibly major source of error in the estimates. By studying the migration of tagged cod in the spawning grounds of Lofoten and Møre between 1975 and 1981, it was found that the main immigration to these spawning grounds is compleeted by March 20 and that considerable emigration starts around April 10. It is thus concluded that the most facourable time to estimate the spawning stock, when taking into acount migration alone, is between these dates

Age-length relationship in coastal cod (Gadus morhua L.) from catches at the Møre coast

Data from cod taken by Danish seine and fish-pots off Sunnmøre and Nordmøre in the years 1969, 1979 and 1980 are analysed. Three groups of coastal cod living separately, was found at Sunnmøre: The first consisted of mature cod (mainly 4 years and older), the second of big immature fish (dominated by 3 years old fish) and the third of small cod (2-3 years old). Some overlapping in the age distributions between the various groups were observed. However, the mean length of an yearclass decreased from group one to group three. Consequently the size more than the age is supposed to be the decisive grouping factor. The lowest mean length for the various age groups were observed at Nordmøre. A higher concentration on the small-fish-size groups of the fish-pot fishery compared with the Danish seine fishery are suggested the most likely reason