Ecology of the euphausiidsThysanoessa raschi,T. inermis andMeganyctiphanes norvegica in Ísafjord-deep,northwest-Iceland

The seasonal abundance, distribution, maturity, growth and population dynamics of the euphausiidsThysanoessa raschi (M. Sars, 1864),T. inermis (Krøyer, 1846) andMeganyctiphanes norvegica (M. Sars, 1857) were studied in Ísafjord-deep, a fjord in northwest Iceland, from February 1987 to February 1988. Sampling was made at nine stations along the length of the fjord at approximately monthly intervals, along with hydrographic measurements and water sampling for nutrient analysis and measurements of chlorophylla concentrations. Spring warming of the water began in late May and maximum temperatures (8° to 10°C) were observed in late July–September. The phytoplankton spring-bloom started in early April, and the highest chlorophylla levels were measured in early May (7.0 mg m^–3). A small increase was observed in the chlorophylla content in August. The greatest abundance of juveniles and males and females of all three species was observed during January and February 1988, during which period the euphausiids were concentrated in the middle and inner parts of the fjord. Euphausiid eggs were first recorded in the plankton in mid-May, and the greatest abundance ofThysanoessa spp. larvae occurred at the end of May. Larvae ofM. norvegica were not observed in Ísafjord-deep, indicating that recruitment of this species was occurring from outside the fjord.T. raschi andT. inermis had a life span of just over 2 yr; the life span ofM. norvegica was more difficult to determine. Almost all femaleT. raschi were mature at the age of 1 yr, while mostT. inermis females appeared not to mature until 2 yr of age. Most males of both species took part in breeding at 1 yr of age. The maximum carapace length ofT. raschi andT. inermis was 8 to 9 and 9 to 10 mm, respectively. The largestM. norvegica had a carapace length of 9 to 10 mm. The spawning of the euphausiids in Ísafjord-deep appeared to be closely related to the phytoplankton spring bloom; water temperature appeared to have no influence on spawning.