Early lifehistory of coastal cod : Along-coast and in-fjord variation in body length, feeding, condition and macro parasites of post settlement juvenile cod (Gadus morhua) in eelgrass (Zostera marina) habitats at the Skagerrak coast.

• Main Author: Schulze, Per-Erik
• Other Authors: Stenseth N.C., Moland E.M. and H.Knutsen.
• Format: Master Thesis
• Language: English
• Published: 2006
• Subjects: marinbiologi fiskeøkologi kysttorsk yngel føde cladocerer lokale bestander kondisjon larveinndrift lengdeforskjeller rekruttering leverindeks bunnslåing lus; VDP::480; Fulton; Gadus morhua; Copepods
• Online Access: http://hdl.handle.net/10852/11645; http://urn.nb.no/URN:NBN:no-14001

Post settlement cod (3-11cm total body length) juvenile s diet, condition, body length and macro parasites were investigated in coastal Skagerrak, June 2005. They were collected by beach seining in eelgrass habitats. Three fjord systems were targeted in eastern Skagerrak and three in the west. Two major exposure regimes were chosen; inner enclosed areas, and outer locations close to the coastal current. The aims were to get baseline data on the early life history of Skagerrak cod, and to investigate possible factors contributing to the records of poor recruitment to cod stocks of the eastern Skagerrak, and of smaller body lengths of juveniles inside many of the fjords. A total of 997 0-group cod were investigated in a balanced study design for length, and of these 796 for external parasites, 363 for condition, liver index and internal parasites, and 218 for stomach contents. Cladocerans were surprisingly the dominant prey item by volume across most sites. A range of other prey types were also common at some sites, among them polychaetes, small and large copepods, small benthic crustaceans, and chironomid larvae. All the post settlement cod juveniles had what is regarded as an acceptable liver index indicating decent nutrient surplus from last weeks feeding. There was a distinct difference in the parasite fauna of the cod juveniles at the inner versus the outer sites: lice (Caligus elongatus) dominating the inner fjord samples in prevalences up to 20%, and nematodes and spiny-headed worm (Echinorhynchus gadi) dominating the outer fjord areas. The study shows that cod recruits settling in Skagerrak coastal habitats already soon after settlement, at 3-4 cm total body length, utilize a wide range of food sources largely decoupled from the North Sea oceanic system. We suggest that the known stable weather conditions and high local coastal production in the early post settlement period of June is a reliable and important food source for the recruits of coastal cod. Abundances were lowest in the east, and consistently lowest at inner sites across all fjords. Low abundances at these sites is suggested to reflect recruitment limitation as they were five times lower than in outer western sites where densities of 0.2 individuals per square meter is close to the saturation point for eelgrass found in other studies. The low abundances can be driven by the known absence of local recruitment in some of the fjords, and is also evidence of restricted import of external recruits to these inner fjords. Identical liver index at inner and outer sites in every fjord indicated that inner fjords were not detrimental environments. Poor environmental profitability is hence an unlikely explanation of the observed trend of shorter and slimmer (lower Fulton K at length) cod juveniles inside the western fjords. A more parsimonious explanation, which also lends support from the pronounced dichotomy in the parasite fauna between inner and outer sites, is a separation in pools of settling larvae between these areas. The presence of transparent juveniles inside the fjords indicated recent pulses of settlement here around 1st of June. Estimated settlement dates along the outer coast were at least three weeks before. This result is in accordance with cod post settlement juveniles inside the fjords being primarily of local origin, and a majority of the ones at the outer coast being recruited from upstream sources like the North Sea or offshore banks with earlier spawning, a picture also emerging from several other recent studies along this Skagerrak coast. Further analysis of the detailed dynamics in the recruitment and early life stages of coastal cod should be very aware of this potentially mixed origin, and also that knowledge from the wealth of studies on the early life history and feeding on large copepods in the oceanic cod stocks seems to have limited relevance for cod settling in coastal areas.