ALBERT

Description: Payne, M. R., Hatfield, E. M. C., Dickey-Collas, M., Falkenhaug, T., Gallego, A., Gröger, J., Licandro, P., Llope, M., Munk, P., Röckmann, C., Schmidt, J. O., and Nash, R. D. M. 2009. Recruitment in a changing environment: the 2000s North Sea herring recruitment failure. – ICES Journal of Marine Science, 66: 272–277. Environmentally induced change appears to be impacting the recruitment of North Sea herring (Clupea harengus). Despite simultaneously having a large adult population, historically low exploitation, and Marine Stewardship Council accreditation (implying sustainability), there have been an unprecedented 6 sequential years of poor juvenile production (recruitment). Analysis suggests that the poor recruitment arises during the larval overwintering phase, with recent survival rates greatly reduced. Contemporary warming of the North Sea has caused significant changes in the plankton community, and a recently identified regime shift around 2000 shows close temporal agreement with the reduced larval survival. It is, therefore, possible that we are observing the first consequences of this planktonic change for higher trophic levels. There is no indication of a recovery in recruitment in the short term. Fishing mortality is currently outside the agreed management plan, and forecasts show a high risk of the stock moving outside safe biological limits soon, potentially precipitating another collapse of the stock. However, bringing the realized fishing mortality back in line with the management plan would likely alleviate the problem. This illustrates again that recruitment is influenced by more than just spawning-stock biomass, and that changes in other factors can be of equal, or even greater, importance. In such dynamically changing environments, recent management success does not necessarily guarantee future sustainability.

Description: There is evidence that the importance of the different spawning grounds of North Sea autumn-spawning herring has changed. It has been hypothesised that as herring stocks collapse, the diversity of spawning sites also collapses. This was found to be the case in the Sea autumn-spawning herring, which collapsed in the late 1970s. The ICES International Herring Larval Survey has been carried out since 1972 and covers most of the potential and historic spawning grounds of herring. recovery of the stock did take place as predicted in terms of biomass, and re-colonisation of old spawning sites also did occur. We show that, despite the delayed response in re-colonisation of the southern spawning areas, there is almost no change in the number of spawning locations where the highest abundances of larvae (top 50%) are found from collapse to recovery (approximately 9 sites). It was a change in these core sites and the spread to other locations with lower larval abundance that caused the spread of herring spawning. We show that larval surveys are a useful tool for describing the dynamics of sub-stock structure in heterogeneous populations such as herring.

Description: The abundance of the sardine in the North Sea suddenly increased after 1995. Since 2002, the sardine has been spawning regularly in the German Bight, and all its life stages can be found in the area. The larval feeding ecology of two small pelagic clupeiform species with very similar life histories was investigated, the particular aim being to determine signs of food overlap. The distribution and feeding of sprat and sardine larvae were investigated during late spring 2003 on two transects covering a wide range of environmental conditions in the German Bight. Larvae co-occurred at all the stations investigated. Sprat and sardine larvae shared a wide range of prey types. Gut fullness and feeding success were similar in both species; however, potential food competition is avoided to some extent by different habitat preferences.

Description: Environmentally induced change appears to be impacting the recruitment of North Sea herring (Clupea harengus). Despite simultaneously having a large adult population, historically low exploitation, and Marine Stewardship Council accreditation (implying sustainability), there have been an unprecedented 6 sequential years of poor juvenile production (recruitment). Analysis suggests that the poor recruitment arises during the larval overwintering phase, with recent survival rates greatly reduced. Contemporary warming of the North Sea has caused significant changes in the plankton community, and a recently identified regime shift around 2000 shows close temporal agreement with the reduced larval survival. It is, therefore, possible that we are observing the first consequences of this planktonic change for higher trophic levels. There is no indication of a recovery in recruitment in the short term. Fishing mortality is currently outside the agreed management plan, and forecasts show a high risk of the stock moving outside safe biological limits soon, potentially precipitating another collapse of the stock. However, bringing the realized fishing mortality back in line with the management plan would likely alleviate the problem. This illustrates again that recruitment is influenced by more than just spawning-stock biomass, and that changes in other factors can be of equal, or even greater, importance. In such dynamically changing environments, recent management success does not necessarily guarantee future sustainability.

Description: This thesis shows different aspects of the distribution of zooplankton and the resulting influence on larval fish survival. The vertical distribution of the key calanoid copepod species in the Bornholm Basin (Baltic Sea), Pseudocalanus acuspes (Giesbrecht 1881), Temora longicornis (Müller 1792), Acartia bifilosa (Giesbrecht 1882), Acartia longiremis (Lilljeborg 1853) and the only cyclopoid copepod Oithona similis (Claus 1863), were investigated. Sampling was carried out with multiple opening/closing nets. The results showed distinct seasonal differences in the vertical distribution for all species. For P. acuspes an ontogenetic migration was detected, with the younger stage dwelling near the surface and the older stages moving successively deeper. The adults inhabited the water layers around the halocline. In November the vertical distribution disintegrated and all stages were distributed over the whole water column. However in this period the older stages (C4, C5) dominated the community. A diel vertical migration was observed for T. longicornis in the summer month (June 2001, July and August 2002). The migration was most distinc for the older stages, moving from 20 metre at night to almost 60 metre at day. No diel migration was observed in April and November. For Acartia species an extended daily migration was not observed, even though the weighted mean depth of Acartia longiremis in July shifted from 10 metre at night to 20 metre at day. Both Acartia species inhabited the water column above the thermocline in spring and summer. However, in November the distribution was broader for all stages in both species. All stages of O. similis inhabited the water layers around the halocline irrespective to the season, showing no vertical migration. The distribution of reproducing female P. acuspes was investigated with the Video Plankton Recorder, an in situ imaging device. The distribution was closely related to high salinity although low oxygen values were avoided. Based on images of female P. acuspes carrying egg sacs the number of eggs per clutch was calculated and using the temperature dependent development time an in situ egg production rate was estimated. Albeit the production was the same between years, the naupli to female ratio was higher in 2003 leading to the conclusion that the naupli mortality might have been reduced. A simple individual based model of P. acuspes connected to a circulation model of the Baltic Sea was used to reveal possible influence of drift to biomass changes. The results showed that in contrast to the 1980’s during the 1990’s the prevailing barotrophic circulation forced the majority of P. acuspes out of the Bornholm Basin into easterly Basins and shallow coastal regions. However, the transport of organisms produced in the eastern Basins (Gotland Basin and Gdansk Deep) back to the Bornholm Basin was limited. Lower salinity and higher temperature in the near shore water might influence the survival and reproduction. The same circulation model was used to investigate the drift of larval cod and possible consequences on the overlap with food organisms. Again, a shift in the baroclinic circulation between the 1980’s to the 1990’s led to an increased dispersion of larvae in the latter period. It was shown that this decreased the overlap with larval prey and thus decreased the survival probability of larval cod. Furthermore, the influence of variability in prey abundance and diversity on the results of an individual based model on larval cod growth and survival was investigated. It was shown, that the use of idealized mean prey fields led to an overestimation of larval survival. Using field data derived from net sampling the survival was partly reduced to 50%. Another investigated factor was the size of prey organisms. Again, the use of mean value led to an overestimation of survival. Using measurements from samples reduced the survival up to 75%.