Atlantic herring (Clupea harengus) in the Irish and Celtic Seas; tracing populations of the past and present
Several herring stocks are found around Ireland, of which the Irish and Celtic Seas are two. Herring in the Irish Sea spawn in autumn and in the Celtic Sea spawning takes place in autumn and winter. During the first year of life Celtic Sea herring disperse into the Irish Sea. The two stocks mix at nursery grounds in the Irish Sea, hampering the production of juvenile abundance estimates for the Irish Sea stock. In this study otolith shape was used to classify juvenile herring at Irish Sea nursery grounds as autumn spawned (most likely of Irish Sea origin) and winter spawned fish (most likely of Celtic Sea origin), with a success rate of 86-87%. Otolith shape analysis separated winter spawned juveniles that are retained in the Celtic Sea from those that disperse into the Irish Sea with a high degree of accuracy (>95%). Shape analysis of the first winter ring in the otolith was then used to determine nursery ground of origin in two year old adult Celtic Sea herring, showing that approximately 42% of fish spent their nursery phase at Irish Sea nursery grounds. This provides strong evidence that Celtic Sea herring, which disperse into the Irish Sea as larvae, return to the Celtic Sea as adults by natal homing. The implications of these findings for herring population structure theories are discussed Otolith microstructure analysis of historical otoliths, collected in the Irish Sea from 1993-2003, revealed interannual fluctuations in the relative proportions of autumn and winter spawned juveniles. The contribution of winter spawned fish (most likely of Celtic Sea origin) was removed from the estimate of juvenile abundance for the Irish Sea stock. This adjustment significantly improved the relationship between juvenile abundance at age-1 and the abundance of age-3 fish from the same year class (r > 0.8, p < 0.05). This separation method could be routinely used to generate recruitment indices for the Irish Sea stock. The abundance of winter spawned juveniles in the Irish Sea was negatively correlated with the frequency of strong south westerly winds during the larval phase (r = -0.96, p<0.05). The underlying cause of this relationship can only be speculated; it may reflect the dispersal of Celtic Sea larvae out of the Irish Sea or increased larval or juvenile mortality during years of frequent strong south westerly winds.
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