Food and feeding habits of young saithe, Pollachius virens (L.), on the coast of western Norway

The stomach contents of young saithe (both I- and II-group, but mainly II-group) from two areas, denoted A and B, on the western coast of Norway have been analysed. When these juvenile saithe become two or more often three years old, they migrate from the coastal shallows to the North Sea. Nutrition may be an important factor governing or influencing this migration. In area A, the saithe had preyed almost exclusively upon plankton in the pelagic environment. The appendicularian Oikopleura dioica, the copepod Calanus finmarchicus, and the krill Thysanoessa inermis each dominated the nutrition at times. In the winter, when krill seemed to be the most important single food organism, the saithe showed clear signs of starvation. In area B, Calanus finmarchicus was the most common single prey. In this area the diversity of the stomach content was higher than in area A. Epifauna and hyperbenthos, consisting of isopods and amphipods attached to bottom vegetation, seemed to play an important role, especially when typical suitable plankton organisms were scarce. Larvae and yearlings of fish were at certain times important food for the saithe. Different species occurred in the diet at different times. Small differences in the length distribution throughout the sampling period are indicative of a gradual migration of the largest fish, mostly two-year-olds, away from the coast. In July-August almost all two-year-olds saithe disappeared from area B for a short time, while they were still present in area A. This behaviour did not seem to appear every year in the investigated areas. Intraspecific competition from younger age groups may be an explanation. The main migration of saithe away from the coast seems to take place during spring (after February, but before June) when the saithe are three years old. In the beginning of March, the saithe seemed to have preyed mostly on krill, and the observed transport of krill with the water masses away from the coast may have led the saithe to follow

Short note on abundance of SEBASTES Mentella in the 0-group and youngfish surveys as indicator of recruitment overfishing

Precision and relevance of pre-recruit studies for fishery management related to fish stocks in the Barents Sea and adjacent waters. Proceeding of the sixth IMR-PINRO Symposium Bergen, 14-17 June 1994Abundance indices of 0-group Sebastes mentella from the international 0-group survey in the Barents Sea showed a sudden decrease from 1990 to 1991 to a low level which remained for three years. The strength of these weak yearclasses has been confirmed in later youngfish surveys, and is supposed to be a result of a historic low spawning stock. A big directed fishery in former years, and huge by-catches and discards of small Sebastes mentella in other fisheries have due to the late maturity of this species not until now resulted in a serious reduction of the spawning stock, A directed trawl fishery in more recent years on mature fish on new fishing grounds has probably speeded-up this negative development

Pre-recruit studies of the North-East Arctic Greenland halibut stock

Precision and relevance of pre-recruit studies for fishery management related to fish stocks in the Barents Sea and adjacent waters. Proceeding of the sixth IMR-PINRO Symposium Bergen, 14-17 June 1994Abundance indices of 0-group Greenland halibut (Reinhardtius hippoglossoides) from the international 0-group surveys in the Barents Sea showed a sudden decrease from 1987 to 1988 to a low level which has remained for the last 5-6 years. The strength of these weak yearclasses has been confirmed in later youngfish surveys, and this has caused strong regulations of the fishery at a time when the fishery itself was good. The drop in recruitment is discussed and related to a reduction of the spawning stock

Stock characteristics, fisheries and management of Greenland halibut (Reinhardtius hippoglossoides Walbaum) in the northeast Arctic

Greenland halibut (Reinhardtius hippoglossoides (Walbaum)) are widely distributed over wide geographic areas of the Northeast Atlantic Ocean, with no break in the continuity of the distribution from the arctic Frans Josef Land and Novaya Zemlya archipelagos in the north and east to beyond the boreal Shetland Islands in the south. Although the entire Greenland halibut resource in the North Atlantic is genetically homogeneous, they comprise a single interbreeding stock in the Barents Sea and Norwegian Sea areas, which is known as the Northeast Arctic stock. In general, and for most of the year, larger fish become more abundant and smaller fish less abundant in progressively deeper water with peak abundance occurring over a depth range of 400-1 000 m. But during the spring and summer, the mature and bigger fish may be shallower. Greenland halibut in the Northeast Atlantic were observed to be most abundant in bottom temperatures mainly between 0º C and 4º C. The fishery for Greenland halibut in the Northeast Arctic was unregulated until 1992, although since 1995 catches have substantially exceeded those advised. The spawning stock size reached historically low levels during the 1990s and recruitment to the spawning stock remained uncertain. The stock is now showing clear signs of improvement, and is at present rebuilt to above the long-term average of the past 20 years

Pre-recruit studies of the North-East Arctic Greenland halibut stock

Abundance indices of 0-group Greenland halibut (Reinhardtius hippoglossoides) from the international 0-group surveys in the Barents Sea showed a sudden decrease from 1987 to 1988 to a low level which has remained for the last 5-6 years. The strength of these weak yearclasses has been confirmed in later youngfish surveys, and this has caused strong regulations of the fishery at a time when the fishery itself was good. The drop in recruitment is discussed and related to a reduction of the spawning stock

Undersøkelse av torsk, hyse og uer i Barentshavet vinteren 1990

The combined acoustic and bottom trawl survey in the Barents Sea in winter has been carried out since 1981. The target species are cod and haddock, but in recent years the redfish species in the area have also been included. In 1989, rock hopper gear was introduced in the bottom trawl survey. This gives considerably higher catchers of the smallest individuals than the bobbins gear. A double set of indices are presented for the bottom trawl survey, one for bobbins gear and one for rock-hopper gear, with indices back-calculated ffrom one gear to the other in the appropriate years. For cod, the acoustic survey gave a total estimate in numbers which was 73 % higher than in 1989, and the bottom trawl survey (rock-hopper gear) a total 16 % higher than in 1989. The corresponding figures for haddock are 85 % and 169 %. The increase is in both surveys largely due to increase in the number of 1-group fish. Thus, the 1989 year class of both species appears to be stronger than the 0-group survey in August-September indicated, and may be of average abundance. Otherwise, the results seem to be in reasonably good agreement with the predictions of stock numbers at age of cod and haddock in the last assessment. For redfish, the bottom trawl indices have not been corrected for the change from bobbins to rock-hopper gear and the effect of the change is not known. The abundance indices from both surveys indicate that the stock situation for Sebastes marinus and Sebastes mentella is stable within the investigated area. The indices for Sebastes viviparus increased in 1990. Indices for the smallest length group are considered unreliable for all three species

Relative selectivity in trawls, longline and gillnets on Greenland halibut

Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-24 June 1997.Selectivity parameters for Greenland halibut (Reinhardtius hippoglossoides, Walbaum) are compared to catches reported fiom trawl, gillnets and longline in the Norwegian scientific fisheries for Greenland halibut. A trouser trawl selectivity experiment reported here gives an L50 at 43 cm in 135 mm codend. A selectivity analysis of the gillnets using loglinear models is done, and show maximum retention probability for lengths at 40.6 - 63.8 cm for the five mesh-sizes used. The effect of the fishiig strategy is analysed in respect to the selectivity of the gear used and the diskibution of length and age in the catches. To avoid possible bias from strong dominating yearclasses and selection in these comparisons, length-at-age data are used. The sex-ratio in gillnet catches is shown to be a linear fiinction of meshsize. Our data show no trend in length distribution with depth. We show that calculated growth of female Greenland halibut is affected by the selectivity of the gears. It is shown that growth parameters calculated fiom gillnet catches may be biased due to the selection properties in the gillnets. These analyses will provide a better understanding of possible sarnpling bias when sampling a stock with only one gear

Estimates of stock size of northeast arctic cod and haddock, Sebastes mentella and Sebastes marinus from survey data, winter 1989

The combined acoustic and bottom trawl survey in the Barents Sea in winter has been carried out since 1981. The target species are cod and haddock, but in recent years the redfish species Sebastes marinus and Sebastes mentella have also been included. Since 1982, an acoustic survey for spawning cod in the Lofoten area has been carried out immediately after the Barents Sea survey. For cod, the acoustic survey gives a total estimate in numbers which is about 10% lower than in 1988, whereas the bottom trawl survey gives a reduction of about 40%. The difference between the surveys probably to some extent reflects a change in the vertical distribution. Poor recruitment is confirmed by both surveys. A similar difference between the surveys is found for haddock, but the reduction in number is larger, about 30% and 60%, respectively, for the acoustic survey and the bottom trawl survey. Abundance indices for redfish indicate that the stock situation is stabilizing for Sebastes marinus and Sebastes mentella, although at a low level. The results of the Lofoten survey indicate that recruitment to the spawning population of cod is improving

Age readings of Sebastes marinus otoliths: bias and precision between readers and otolith preparation methods

This study presents a comparative age reading on Sebastes marinus from the Icelandic shelf within an otolith exchange program between institutes in Germany, Iceland and Norway. Out of a series of 212 otolith pairs, one otolith of each pair was prepared by the break-and-burn technique, while the other otolith was used for crosssections. Age reading results are compared between readers and otolith preparation methods in terms of bias and precision, using a set of statistical tests and graphical methods. Significant bias was observed for both the comparison between readers and between methods, mainly caused by deviations between age scores in the higher ages (> 20 years). Precision estimates, involving the high longevity of redfish, were relatively good compared to previous age reading comparisons using other species. In contrast, the age dependent percent agreement was poor (< 30%) for a tolerance level of ± 0 years, particularly for the age range 21-30 years. A tolerance level of ± 3 years, however, lead to around 90% agreement for the age range up to 20 years. The fit of age reading scores with the von-Bertalanffy growth curve was relatively good, providing growth parameters comparable to S. marinus from the Norwegian shelf. The observed problems in bias and precision of age readings should to be improved by continuing with similar Sebastes otolith exchange programs and setting up a further age reading workshop to harmonise the interpretation of growth structures