Tracing populations of Atlantic herring (Clupea harengus L.) in the Irish and Celtic Seas using otolith microstructure

Abstract

Otolith microstructure of juvenile herring was used to distinguish autumn- and winter-spawned fish collected in the Celtic and Irish Seas in 1999 and 2000. Juveniles showed two distinct patterns of larval otolith growth; widely spaced increments (fast growers) and tightly packed increments (slow growers). Fast- and slow-growing fish exhibited a clear bimodal distribution and the two groups were completely distinct at increments 60–70. Otolith increment counts confirmed that the fast-growing fish originated from the winter spawning season and that slow-growing fish were spawned in autumn. This characteristic pattern of otolith growth was used to determine the relative proportions of autumn- and winter-spawned fish at nursery grounds in the Irish and Celtic Seas. Although the proportions in each area varied between years, nursery grounds in the western Irish Sea were generally dominated by winter-spawned fish whereas autumn-spawned fish were mostly at eastern stations. Given the distribution of spawning adults in the region, the vast majority of winter-spawned fish are likely to originate from the Celtic Sea. Larval growth differences between nursery areas suggest that Celtic Sea herring drift into the Irish Sea during the larval phase. There was significant variation in fish size between nursery areas for both autumn- and winter-spawned fish. Juvenile length was greatest in the Celtic Sea, intermediate in the eastern Irish Sea, and lowest in the western Irish Sea. Therefore, larval dispersal of Celtic Sea herring into the Irish Sea produces components in the surviving juvenile population with characteristic growth patterns. This population segregation could potentially impact on subsequent growth, survival and recruitment.