Spatial variation in hatch date distributions and origin of pelagic juvenile cod in Icelandic waters

Hatch date distributions, local spawning time and incubation temperatures at different spawning sites around Iceland were used to trace the most likely origin of 0-group cod. Length, age and hatch date frequency distributions were obtained from young larvae collected on the main spawning grounds off the south coast in 1995 and from pelagic 0-group cod captured in six nursery regions off the west, north and east coast of Iceland in 1995, 1996 and 1997. Historically, the 0-group cod found in coastal areas north and east of the country are believed to have originated from the main spawning grounds at the south-southwest coast. Contrary to the expected dispersal pattern, both size and age of 0-group cod decreased from west to north, when moving clockwise around the country away from the main spawning grounds in the south. It is shown that this spatial variation in size is not explained by different growth rates but by age. Consequently, the reconstructed spawning periods of surviving 0-group cod captured along the north and the east coast and the recorded spawning time for cod on the main spawning grounds in the south were not congruent. Large numbers of these 0-group cod, especially in 1995 and 1996, appeared to have hatched in June and July or considerably later than the recorded time period of hatching on the main spawning grounds in April and May. Therefore, rather than originating on the main spawning grounds in the south, some of the surviving 0-group cod originated from spawning that occurred in the western, northern and eastern fjords, during either the spring when local water temperatures were 3–4 C colder, or in early summer at a much later time. Spawning females have been observed at these locations over an extended time period (April–June) and protracted incubation periods due to cold waters will result in delayed hatching of larvae. In some years the smaller northern spawning sites, may contribute significantly towards the surviving population of 0-group juveniles, and probably contributing to the relative stability in recruitment experienced by this stock.