The distribution and abundance of sand gobies, Gobius minutus, in the Ythan estuary

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Abstract
The aims of this study were, first, to describe the distribution and abundance of the sand goby (Gobius minutus) in the Ythan estuary and, when it became apparent that there were striking seasonal changes in the abundance of gobies, to attempt to explain these changes.Gobies were abundant in the estuary from July until January or February each year, and scarce for the rest of the year. The sharp drop in goby numbers in the estuary in January or February each year was associated with a change in the sex ratio from 50% females to 80% females, and a change in dispersion from aggregated to random, and a restriction of the gobies’range to the lower half of the estuary. It seemed to me that these associated changes should all be the result of a common cause.The disappearance of gobies in January or February was not a result of low water temperatures in the estuary in winter, nor was it the result of changes in water salinity. Measurements of food intake and available food supplies suggested that there was no shortage of food at this time. The disappearance was, however, associated with gonad maturation, suggesting that the loss of fish had something to do with the breeding requirements of the goby. I put forward the hypothesis that the loss of gobies in winter was the result of their migrating to sea to breed because their eggs would not develop in the estuary. I tested this hypothesis by attempting to capture gobies at sea, by measuring their salinity preference, and by attempting to rear their eggs at various salinities. I was unable to capture more than a few gobies outside the estuary. Ripe fish did not show any preference for high salinity water, and their eggs were intolerant of high salinities. It seemed, therefore, that my hypothesis of an emigration to sea to breed was incorrect, and that something else must explain the losses of gobies in winter, possibly heavy predation.

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