Description: The influence of ambient temperature (2-7°C) and oxygen level (1.0-8.3 ml· l- 1) on the development of Baltic cod eggs was investigated in laboratory experiments. The incubation period, i.e. the time from fertilization to 50% hatching, decreased from 27.5 days at 2°C to 13.0 days at 7°C. Reduced oxygen levels did not significantly affect the time of hatching. Throughout the incubation period highest mortality rates were found during gastrulation and immediately prior to hatching at all tested oxygen levels. Egg survival decreased from around 30% at an oxygen level of 8 ml· 1- 1 to less than 10% at 2 ml· 1- 1 oxygen content. At oxygen concentrations below 2 ml 0 2 • l- 1 the development ceased at a very early stage. Field observations revealed that in the past years Baltic cod eggs were most abundant below the halocline, depth with unfavourable oxygen condition. Besides the effect on egg survival, low environmental oxygen may also affect the initial viability of larvae and consequently their ability to approach the feeding areas close to the sea surface. Thus, the effective reproduction volume of water for cod in the central Baltic may have been smaller than expected and it is suggested that oxygen depletion was the limiting factor determining the reproductive success of cod in this area during the last decade.

Description: Abstract The Boreoatlantic gonate squid Gonatus fabricii is the most abundant squid in the offshore arctic and subarctic waters of the northern Atlantic. Adults are common in midwater while juveniles occur in surface waters close to the continents. As part of a research project focussing on the interactions among fish stocks off West Greenland we examined squid collections sampled with small pelagic nets in summer 1989 and in summer and autumn 1990 off Southwest Greenland. G. fabricii was by far the most abundant cephalopod species caught. We recorded a total of 698 juvenile specimens. During the summer cruise in 1989 the mantle lengths varied from 6 to 35 mm (n = 84); in summer 1990 they ranged from 10 to 48 mm (n = 542) with significantly larger body sizes in the southern part of the region. In autumn 1990 the mantle lengths ranged from 19 to 64 mm (n = 72) with largest animals again at the southern sampling sites. The data suggest growth rates for juvenile G. fabricii off West Greenland of 4 to 5.5 mm per month between July and November 1990. Résumé L'encornet atlantoboréal (Gonatus fubricii) est l'encornet pélagique le plus abondant des eaux arctiques et subarctiques de l'Atlantique. Les adultes se trouvent habituellement dans les couches de profondeur moyenne, et les juvéniles se trouvent en surface et proche de zone littorale. Parallèlement à une étude des stocks de poissons à l'ouest du Groënland, nous avons étudié les encornets pris avec des petits filets pélagiques en été 1989 et en été et automne 1990. G. fabricii fût le céphalopode le plus abondant, avec 698 juvéniles échantillonnés. En été 1989, les longueurs du manteau étaient de 6 à 35 mm (n = 84); en été 1990, les longueurs étaient de 10 à 48 mm (n = 542), et la taille du corps était significativement plus importante au sud de la région échantillonnée. En automne 1990, les longueurs du manteau étaient de 19 à 64 mm (n = 72), et les animaux les plus grands se trouvaient encore dans le secteur sud. Les données suggèrent un taux de croissance d'entre 4 et 5,5 mm par mois pour les juvéniles de G. fabricii sur la côte sud-ouest de Groënland pour la période de juillet à novembre 1990.

Description: Estimates of the reproduetion volume of Baltic eod defined as the. total volume of water with salinity above 11 psu, oxygen coneentration above 2 mIlI and temperature above 1.5 °c are presented for the Bornholm Basin based on quasi-synoptie hydrographie surveys. The primary data set consisted of hydrographie observations recorded during 14 eruises in the period from 1991 to 1996 eovering a regular grid of 36 stations with a spacing of appr. 10 nm. This data set was supplemented by including two additional surveys from 1989 with 21 stations. A statistical method the objective analysis was applied to mup the horizontal distribution of the thickness of the spawning layer. Subsequently, the reproduetion volume was ealculated by basiri wide integration. Highly significant eorrelations (r ~ 0.89, p 〈 0.001) betwcen single point observations of the thickness of the spawning layer and the overall reproduetion volume in the Bornholm Basin was found for 4 stations loeated in the eentral part of the surveyed urea. Linear regression models for single stations and the basin wide reproduction volume were established representing different hydrographie scenarios from stagnation to inflow periods. The results are discussed with respect to a backward extension of the time series using historical single point observations.

Description: The volume of water with suitable oxygen and salinity conditions for survival and development of cod/eggs in the eastern Baltie Sea varies significantly within and among years. It has recently been shown that this volume of water (nreproduetive volumen), in addition to spawning stock biomass, is a major factor determining recruitment success in this eod population. However, it is unclear which oceanographie mechanisms are responsible for variability in reproductive volume, and how these processes interaet on a seasonal and interannual basis. In this study, we use time series observations to identify inter-relationships between hydrographie and biological processes (e. g., inflows of North Sea water, winter mixing processes, production and decomposition of organic matter) influencing reproductive volumes from 1952-1992. A frequent pattern of seasonal variability in 4 eastern Baltic spawning areas includes a spring maximum which decreases during the summer, followed by an inerease during the fall-winter months. However, this general pattern is violated in many years, and the magnitude of the seasonality is more pronounced, in some spawning areas than others. \Ve have begun to analyse these patterns and interpret them in the context of other fluctuating components of the eastern Baltic pelagic ecosystem. Preliminary results show that reproductive volumes deerease by an average of 9 - 39% between May and August, and that the decrease in the spawning area most important to long-term recruitment (Bornholm Basin) is temperature-dependen1. The seasonal decline in the Bomholm Basin also tends to be larger and more variable after 1964 than in the previous 12-year periode These patterns of reproductive volume variability may be important to eod reproduetive success beeause of a long and variable spawning period.

Description: Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics.

Description: The Boreoatlantic gonate squid Gonatus fabricii is the most abundant squid in the offshore arctic and subarctic waters of the northern Atlantic. Adults are common in midwater while juveniles occur in surface waters close to the continents. As part of a research project focussing on the interactions among fish stocks off West Greenland we examined squid collections sampled with small pelagic nets in summer 1989 and in summer and autumn 1990 off Southwest Greenland. G. fabricii was by far the most abundant cephalopod species caught. We recorded a total of 698 juvenile specimens. During the summer cruise in 1989 the mantle lengths varied from 6 to 35 mm (n = 84); in summer 1990 they ranged from 10 to 48 mm (n = 542) with significantly larger body sizes in the southern part of the region. In autumn 1990 the mantle lengths ranged from 19 to 64 mm (n = 72) with largest animals again at the southern sampling sites. The data suggest growth rates for juvenile G. fabricii off West Greenland (Arctic Ocean) of 4 to 5.5 mm per month between July and November 1990.

Description: A study on cod egg mortality was carried out in the Bornholm Basin (southern central Baltic Sea) toward the end of July 1996. An initial egg aggregation marked by a satellite-tracked drifter buoy was sampled repeatedly over an 11-day period; profiles of temperature, salinity and dissolved oxygen were concurrently recorded. Three replicate estimates of mortality were obtained for each pair of subsequent developmental stages from newly spawned eggs to early larvae. A consistent pattern of stage-specific mortality coincided well with previous experimental observations. Average daily mortality rates were 7.2% (eggs IA/IB), 38.7% (eggs (IB/II), 25.6% (eggs II/III), 40.0% (eggs III/IV), and 42.3% (eggs IV/early larvae). The cumulative mortality until hatch amounted to 99.9%. Results from hydrodynamic modelling, however, indicated that the drifter's trajectory was influenced by wind stress. Hence, the mortality rates might be biased despite the short sampling intervals; a modification of the sampling design is recommended for future studies.

Description: Global studies imply that cephalopods have benefited from climate change. However, in most areas, species-specific long-term cephalopod data sets do not exist to support this implication and to analyse the response of cephalopods to environmental changes. Our results illustrate that historical studies, in combination with recent data sets, can fill this gap, enabling descriptions of ecological changes over a long time. We show substantial changes in the cephalopod biodiversity of the North Sea at species level over the past 100 years. Some species, which seemed to migrate into the North Sea only for spawning or foraging in the nineteenth century, occur permanently in the North Sea nowadays. This applies, for example, to the loliginids Loligo forbesii and Alloteuthis subulata. The ommastrephids Todaropsis eblanae and Illex coindetii, now constantly present as well, had been described only as accidental migrants 100 years ago.