Migration behaviour of norwegian spring spawning herring when entering the cold front in the Norwegian Sea

Survival and local recruitment are driven by environmental carry-over effects from the wintering area in a migratory seabird

We estimated annual apparent survival rates, as well as local recruitment rates in different age groups and for different breeding status in the common tern Sterna hirundo using mark-recapture analysis on a long-term individual-based dataset from a breeding colony in Germany. Strong inter-annual variability in survival rates became apparent, especially in prospectors. Local recruitment also varied strongly between years and age groups. To explain these fluctuations, we linked survival and recruitment estimates to several environmental covariates expected to be limiting during the wintering period and migration, including the global climate indices of North Atlantic Oscillation and Southern Oscillation, fish abundance indices, and marine primary productivity in the West African wintering area. Contrary to expectations, global indices did not seem to be linked strongly to vital rates. Results showed that primary productivity had the strongest effect on annual survival, especially in young and inexperienced individuals. Primary productivity in the wintering area was also strongly associated with the probability of recruitment in the following breeding season, indicating that conditions during winter can have carry-over effects on the life cycle of individuals.

Meta-ecosystems and biological energy transport from ocean to coast: the ecological importance of herring migration

Ecosystems are not closed, but receive resource subsidies from other ecosystems. Energy, material and organisms are moved between systems by physical vectors, but migrating animals also transport resources between systems. We report on large scale energy transport from ocean to coast by a migrating fish population, the Norwegian spring-spawning (NSS) herring Clupea harengus. We observe a rapid body mass increase during parts of the annual, oceanic feeding migration and we use a bioenergetics model to quantify energy consumption. The model predicts strong seasonal variation in food consumption with a marked peak in late May to July. The copepod Calanus finmarchicus is the most important prey and 23 x 10(6) tones (wet weight) of C. finmarchicus is consumed annually. The annual consumption-biomass ratio is 5.2. During the feeding migration 17% of consumed energy is converted to body mass. The biomass transported to the coast and left as reproductive output is estimated from gonad weight and is about 1.3 x 10(6) tones for the current population. This transport is to our knowledge the world's largest flux of energy caused by a single population. We demonstrate marked temporal variation in transport during the last century and discuss the effects of NSS herring in the ocean, as a major consumer, and at the coast, where eggs and larvae are important for coastal predators. In particular, we suggest that the rapid decline of lobster Homarus gammarus landings in Western Norway during the 1960s was related to the collapse of NSS herring. We also discuss spatial variation in energy transport caused by changed migration patterns. Both climate and fisheries probably triggered historical changes in the migration patterns of NSS herring. New migration routes emerge at the level of individuals, which in turn determines where resources are gathered and delivered, and therefore, how meta-ecosystems function.

Herring schooling manoeuvres in response to killer whale attacks

The antipredator behaviour of overwintering Norwegian spring-spawning herring (Clupea harengus L.) was investigated during repeated attacks by killer whales (Orcinus orca L.) in Tysfjord in northwestern Norway. The observations were made using a high-resolution (455 kHz) multibeam sonar. Ten different types of predator-prey interactions were recorded during 54 observed events (an average of one antipredator event every 3.9 min). Antipredator responses included "split," "hourglass," "vacuole," "bend," "dive," "herd," and "fountain." Large attacked schools demonstrated a different repertoire of antipredator manoeuvres than small ones and were less likely to be attacked. Despite being located in the vicinity of the whales, herring schools with a cross section exceeding 460 m2 were not attacked by killer whales. Attacked schools were significantly more circular (p < 0.0001) and had higher relative densities (p < 0.05) than schools that were not attacked.