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

Abstract

Abstract. The seasonal abundance, distribution, maturity, growth and population dynamics of the euphausiids Thysanoessa raschi (M. Sars, 1864), T. inermis (Kroyer, 1846) and Meganyctiphanes norvegica (M. Sars, 1857) were studied in Isafjord-deep, a fjord in northwest Iceland, from February 1987 to February 1988 Sampling was made at nine stations along the length of the f~ord at approximately monthly intervals, along with hydrographic measurements and water sampling for nutrient analysis and measurements of chlorophyll a 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 chlorophyll a levels were measured in early May (7.0 mg m-3). A small increase was observed in the chlorophyll a 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 of Thysanoessa spp. larvae occurred at the end of May. Larvae of M. norvegica were not observed in [saljord-deep, indicating that recruitment of this species was occurring from outside the fjord. T. raschi and T. inerrnis had a life span of just over 2 yr; the life span of M. norvegica was more difficult to determine. Almost all female T. raschi were mature at the age of 1 yr, while most T. 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 of T. raschi and T. inerrnis was 8 to 9 and 9 to 10 ram, respectively. The largest M. norvegica had a carapace length of 9 to 10 ram. The spawning of the euphausiids in Isafjord-deep appeared to be closely related to the phytoplankton spring bloom; water temperature appeared to have no influence on spawning.

Distributions and seasonal abundances of krill eggs and larvae in the sub-Arctic Godthåbsfjord, SW Greenland

“…Several authors have suggested that spawning of Thysanoessa spp. coincides with the onset of the phytoplankton spring bloom (Einarsson 1945, Berkes 1976, Lindley & Williams 1980, Falk-Petersen & Hopkins 1981, Astthorsson 1990, Gislason & Astthorsson 1995, Dalpadado 2006, Plourde et al 2010, which is further supported by the present results. During the study period, there was a second peak in egg abundance at the beginning of July, indicating a second spawning event, as described by MacDonald (1928) for T. raschii in the Firth of Clyde.…”

“…However, the absence of smaller individuals and non-fertilized females of M. nor vegica suggests that this species does not reproduce in the fjord but is advected from outside the fjord (Agersted & Nielsen 2014). Astthorsson (1990) respectively. The spawning in Kapisigdlit is therefore assumed to be that of Thysanoessa spp.…”

“…Krill eggs increased in abundance during late April and early May, following initiation of spawning. Spawning by species of Thysanoessa has previously been observed in April and May in Iceland (Astthorsson 1990, Gislason & Astthorsson 1995, the Barents Sea (Dalpadado & Skjoldal 1991), the North Sea (Lindley & Williams 1980) and the St. Lawrence estuary (Plourde et al 2010). During April to May, surface water temperature increased from 1°C to approximately 5°C in the inner part of the fjord, and a phytoplankton bloom was apparent (Stn K4).…”

“…Knowledge about the biology of krill larvae and the relationships among their spatial and seasonal distributions and experienced physical and biological parameters is essential to understanding krill population dynamics. This has been intensively investigated for the Antarctic species Euphausia superba (Huntley & Brinton 1991, Flores et al 2012, Gao et al 2013), but much less knowledge is available for the North Atlantic and sub-Arctic species (Astthorsson 1990, Astthorsson & Gislason 1997, Dalpadado et al 2008.…”

Abundance, biomass and life cycle patterns of euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes) in the Oyashio region, western subarctic Pacific

“…It is noted that the spawning season of this euphausiid overlaps the period of the spring phytoplankton bloom both in the central Japan Sea (Chiba & Saino 2002) and in the Oyashio region (Kasai et al 2001). Coincidence of reproduction and the spring phytoplankton bloom has also been reported for other subarctic Thysanoessa euphausiids (T. inermis Krøyer and T. raschii M. Sars) in the Atlantic Ocean (Falk-Petersen & Hopkins 1981, Astthorsson 1990.…”