Jakob Gjøsæter

Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak, southern Norway

The European eel (Anguilla anguilla L.) is distributed in coastal and inland habitats all over Europe, but spawns in the Sargasso Sea and is thus affected by both continental and oceanic factors. Since the 1980s a steady decline has been observed in the recruitment of glass eels to freshwater and in total eel landings. The eel is considered as critically endangered on the International Union for the Conservation of Nature and Natural Resources Red List of species. The Skagerrak beach seine survey from Norway constitutes the longest fishery-independent dataset on yellow/silver eels (starting in 1904). The Skagerrak coastal region receives larvae born in the Sargasso Sea spawning areas that have followed the Gulf Stream/North Atlantic Drift before they penetrate far into the North Sea. The Skagerrak coastal time series is therefore particularly valuable for exploring the impacts of oceanic factors on fluctuations in eel recruitment abundance. Analyses showed that Sargasso Sea surface temperature was negatively correlated with eel abundance, with a lag of 12 years revealing a cyclic and detrimental effect of high temperatures on the newly hatched larvae. The North Atlantic Oscillation index and inflow of North Atlantic water into the North Sea were negatively correlated with eel abundance, with a lag of 11 years. Increased currents towards the North Atlantic during high North Atlantic Oscillation years may send larvae into the subpolar gyre before they are ready to metamorphose and settle, resulting in low recruitment in the northern part of the distribution area for these years. The Skagerrak time series was compared with glass eel recruitment to freshwater in the Netherlands (Den Oever glass eel time series), and similar patterns were found revealing a cycle linked to changes in oceanic factors affecting glass eel recruitment. The recent decline of eels in the Skagerrak also coincided with previously documented shifts in environmental conditions of the North Sea ecosystem.

The food of the myctophid fish, Benthosema glaciale (Reinhardt), from western Norway

Abstract Stomach contents were studied in Benthosema glaciale taken by midwater trawl in a fjord in western Norway. Copepods constitute the most important part of the identifiable stomach contents. Euphausiids were found during all seasons except summer, and more commonly in big than in small fish. The fish feed all the year, but most intensively during spring and summer. They seem to feed more in the evening than during the light hours of the day. The seasonal and diurnal variation in feeding may be related to the availability of food.

Age, growth, and mortality of the myctophid fish, Benthosema glaciale (Reinhardt), from western Norway

Abstract Age and growth of Benthosema glaciale collected by midwater trawl in western Norway have been studied. Otoliths were used for age determination. The mean annual mortality in age groups I to IV was 52 %. Older specimens were sparse. Growth was fastest in winter. Back calculation of growth based on otoliths was tried, but the results deviated from those obtained by direct observations of length and age. B. glaciale is shown to grow faster and to have a higher maximum length in Norway than off Nova Scotia.

Genotype distributions of cod from the Norwegian Skagerrak coast

Abstract The genotype distributions for cod, Gadus morhua L., from the southeastern coast of Norway are studied as part of a more extensive study on cod enhancement and the possible effects of mass liberation of 0-group cod on endemic cod populations. This paper focus on possible geographical variation in allele frequencies, temporal variations in haemoglobin type frequencies and mean length of age of various genotypes. Twelve samples were analysed for genotype distribution of haemoglobins and the tissue enzymes LDH, PGI, IDH, PGM, and GPD by agar-gel and starch gel electrophoresis. Some inters ample variations were found indicating heterogeneity of the total cod stock structure in the area. The haemoglobin-controlling genes occurred with similar frequencies in this material as in corresponding material analysed more than 25 years earlier. Possible associations between life history traits and individual genotypes were investigated, and genotype-dependent growth rate was indicated.

Age and growth of four Sardinella species from Sri Lanka

Abstract Age, growth and spawning season were studied in Sardinella sirm, S. albella, S. gibbosa and S. longiceps collected by gill nets in the coastal waters of Sri Lanka. Ageing was based on primary growth rings in the otoliths. The daily periodicity as verified for S. sirm by comparing mean increase in number of rings and the time elapsed between a series of sampling dates. For the other species, the same periodicity was inferred from the similarity in structure to that of S. sirm. The species reach the following lengths after 1 year: S. albella, 12 cm; S. gibbosa, 13 cm; S. longiceps, 14 cm; S. sirm, 20 cm. The total life-cycles are 1–1.5 years. Apparently, all species reach maturity after about 1 year. There seem to be two spawning seasons per year. Estimates based on counting growth rings indicate that these coincide with the south west and north east monsoonal changes. This also agrees with the changes in maturity stages observed.

Distribution and density of goldsinny wrasse ( Ctenolabrus rupestris ) (Labridae) in the Risør and Arendal areas along the Norwegian Skagerrak coast

Samples of fish were collected in the Arendal and Risør areas on the Norwegian Skagerrak coast during the period 1986 to 1990 using a beach seine at 36 different locations. In the Risør area, 131 out of 254 hauls yielded goldsinnies, and in the Arendal area 66 out of 100 hauls. The density of fish estimated for the summer period (June-September) was as follows: Sandnesfjorden 0.9, Sørfjorden 2.8, Nordfjorden 9.8, Risør Skerries 2.0 and Arendal 12.7 fish 1000 m -2 . The best catches of goldsinny were taken at stations with medium exposure, but the difference in mean catches between stations with different exposure was not significant. The average number of goldsinny taken at stations was not significantly related to freshwater influence, as inferred from the distance to river outlets. The presence of rock in the substratum significantly increased the average number of goldsinny at a station compared to stations without rock. No similar effect was found for the presence or absence of sand or mud. Bottom with vegetation is highly preferred compared to barren bottom.Samples of fish were collected in the Arendal and Risor areas on the Norwegian Skagerrak coast during the period 1986 to 1990 using a beach seine at 36 different locations. In the Risor area, 131 out of 254 hauls yielded goldsinnies, and in the Arendal area 66 out of 100 hauls. The density of fish estimated for the summer period (June-September) was as follows: Sandnesfjorden 0.9, Sorfjorden 2.8, Nordfjorden 9.8, Risor Skerries 2.0 and Arendal 12.7 fish 1000 m -2 . The best catches of goldsinny were taken at stations with medium exposure, but the difference in mean catches between stations with different exposure was not significant. The average number of goldsinny taken at stations was not significantly related to freshwater influence, as inferred from the distance to river outlets. The presence of rock in the substratum significantly increased the average number of goldsinny at a station compared to stations without rock. No similar effect was found for the presence or absence of sand or mud. Bottom wit...

Sarcotretes scopeli, a lernaeid copepod new to Norway

Abstract Sarcotretes scopeli, parasitic on Benthosema glaciale, is recorded for the first time in Norway. 3.6% of the catch from the Bergen area and 1.9% from Sognefjorden was infested. The parasite seems to retard the growth and hinder maturation of the gonads of the host.

New records of the myctophid fish, Notoscopelus kroeyeri, from Norway and the eastern Atlantic

Abstract Four specimens of Notoscopelus kroeyeriare recorded from the south and west coast of Norway, and 30 from the eastern North Atlantic. Some meristic characters are listed, and the distribution of the species briefly discussed.

Fishery for goldsinny wrasse (Ctenolabrus rupestris) (Labridae) with pots along the Norwegian Skagerrak coast

Goldsinny wrasse ( Ctenolabrus rupestris ) was captured in pots during the years 1994-98 in the area between Arendal and Homborsund, west of Grimstad, along the Norwegian Skagerrak coast. The pots, made of steel 1 1 cm square-meshed netting, were 105 cm in length and 31 cm in diameter. They were baited using various crushed crustaceans. The pots were placed at depths between 5 and 10 m on rocky bottom with a dense cover of Laminaria spp. and other macroalgae. A professional fisherman conducted the fishing. Sometimes the pots were hauled once a day, and sometimes twice or three times a day. This did not influence the catch rates. The catch per pot ranked up to 390. The mean catch per pot was 25.8, 18.8, 46.9, 42.7, 55.5 for 1994, 1995, 1996, 1997 and 1998, respectively. Distribution of catch per pot was highly skewed to the left for the first 2 years, while the distribution became progressively more symmetrical for the later years. In 1997, 10% of the pots yielded more than 90 fish, and, in 1998, 9% yielded higher catches. For the other years, this figure ranges from 0.1% to 3%. The proportion of pots yielding fewer than 10 fish decreased from nearly 40% in 1995 to 1% in 1998. The catch rates were dependent on temperature. The data available do not indicate that the fishery had a negative effect on the stock.Goldsinny wrasse ( Ctenolabrus rupestris ) was captured in pots during the years 1994-98 in the area between Arendal and Homborsund, west of Grimstad, along the Norwegian Skagerrak coast. The pots, made of steel 1 2 1 cm square-meshed netting, were 105 cm in length and 31 cm in diameter. They were baited using various crushed crustaceans. The pots were placed at depths between 5 and 10 m on rocky bottom with a dense cover of Laminaria spp. and other macroalgae. A professional fisherman conducted the fishing. Sometimes the pots were hauled once a day, and sometimes twice or three times a day. This did not influence the catch rates. The catch per pot ranked up to 390. The mean catch per pot was 25.8, 18.8, 46.9, 42.7, 55.5 for 1994, 1995, 1996, 1997 and 1998, respectively. Distribution of catch per pot was highly skewed to the left for the first 2 years, while the distribution became progressively more symmetrical for the later years. In 1997, 10% of the pots yielded more than 90 fish, and, in 1998, 9% yiel...

Morphological and ecological studies of Clavella adunca (Copepoda, Lernaeopodidae) on polar cod, Boreogadus saida

Abstract Clavella adunca (STROM, 1762) from the Barents Sea polar cod was studied. The general appearance and morphology of the appendages were similar to C. adunca f. iadda LEIGH-SHARPE, but the size of the trunk and cephalothorax were greater than those in other populations (mean trunk length 3.8 mm, width 3.0 mm, mean cephalothorax length 4.3 mm). Most of the parasites (94 %) were found on the fins. Parasites were found in the whole distribution area of the polar cod, but were most frequent in the eastern part of the Barents Sea. Of 42 samples 39 contained infected fishes. The distributions of the parasites on the hosts were always overdispersed and negative binominal distributions proved to be a good empirical model of the observed distributions. The infection rate was higher during winter than during summer. No correlation between the infection rate and the age of the host was observed.