Recruitment variability in Norwegian spring-spawning herring (&pea harengus L.): the effect of temperature in larval drift trajectories. ICES CM 2000/N:08

During late winter, the Norwegian spring-pawning (NSS) herring migrate southward along the Norwegian coast to spawn in March-April . The larvae drif t northwards in the Norwegian coastal current during their f irst months and eventually most of the larvae and juveniles end up in the Barents Sea. Here, we have compiled measurements of temperature alon, 0 the coast back to 1936 and analyse these data for correlation with recruitment success (recruits/SSB) of the herring. We find that although the average temperature in the larval drift trajectory may vary with as much as 2°C between years, the effect on recruitment success is minor. A model using temperature-dependent growth and size-dependent mortality predicted a strong relationship between recruitment and the annual temperature variat ions, but no strong support of this was evident in the data. Nor did we fmd a relat ion between recruitment success and the condit ion of the spawning stock, while a strong stockrecruitment rdationship and density-dependent recruitment prevails . Introduction Environmenta factors affecting growth and survival of fish larvae are expected to affect the variability of recruitment success for a given Spawning Stock Biomass (SSB). Among the most important variables is temperature, which has a major influence on growth rate in most fish larvae, including Norwegian spring spawning (NSS) herr ing (Fiksen & Folkvord 1999). There are several studies relating averaged temperatures to recruitment (see e.g. Ottersen & L.oeng 2000, Toresen & 0stvedt in press). Naturally, it is difficult to find measures of temperature that overlaps the spatial distr ibution of the larvae over t ime, and consequently these relat ions are typically made by correlating large-scale average temperatures with recruitment. In the analysis presented here, we search for patterns of recruitment success in NSS herring originating from both the condition of the SSB, and the ambient temperature at the time and location where the larvae hatch and their subsequent drift trajectories. Toresen & !&tvedt (in press) describe the stock structure of NSS herring back to 1907, and we applied these data for this study. Sea temperatures from surface to a few hundred meters have been measured at lighthouses along the Norwegian coast since 1936. Most of these observations have been compiled by Aure & 0stensen (1993), and wilI be available from Institute of Marine Research (Bergen, Norway). NSS herring spawn along the Norwegian coast f%om Lista near the southern t ip of Norway to Eggum in Lofoten (Fig. 1). After hatching, the larvae drift northwards in the Norwegian Coastal Current and eventually they end up in the Barents Sea where they stay until 2-3 years old when they join the adult population in the Norwegian Sea. The measurements from the lighthouses thus cover the relevant area for the larvae. Data analysis and modelling spatiotemporal interpolation of femperafure data The Norwegian Coastal Current transports the larvae northwards from their hatching si te along the coast (Fig. 1) . At M#re, some of the larvae may be mixed into the North-East Atlantic Current, and the most fortunate individuals will end up in Norwegian fjords or in the Barents Sea by the end of the year. Temperature measurements from the l ighthouses are made at opportunity, therefore the intervals between each measure vary from several times each week to only a few per year. A few years there is no data available from some of the lighthouses. We have interpolated the data from Lista, Utsira (Western side), Sognesj$?en, Bud, Eggum and Ingay in time and space, such that each day there is a temperature accessible for each 2sLh ki lometre along the coast from Lista to Ingay (Fig. 1) . We assume that the temperature at 10 m depth is the most relevant to herring larvae. The interpolation was linear between each measured point. In Fl,. ‘m 2, the time-interpolated temperatures at each station are displayed for day 91-140 for all 65 years from 1936-2000.