Sequential hermaphroditism, a creature's ability to change sex during its lifetime, has been documented in many commercially and recreationally important species. While this phenomenon is well known, many assessments use the same methods for both stocks that change sex and those that don't. Failing to account for the biological differences in hermaphroditic stocks may lead to poor estimates of biological reference points such as maximum sustainable yield (MSY), and even to the decline of the stock.
Sex change may be triggered in many ways. For some species it may occur at a fixed age or size. These species can be greatly affected by fishing that targets larger individuals. The sex ratio can become heavily skewed towards females, with sperm limitation due to a lack of large males potentially leading to the collapse of the stock. Other sex changes may be environmentally or socially triggered, such as through an individual's size relative to the others in a stock. In this case, the plasticity in the timing of sex change may allow a stock to compensate for the loss of larger individuals, and such a stock may be more resilient to size-selective fishing.
In this study, the researchers evaluated the effects of fishing on black sea bass, a sequential protogynous hermaphrodite (changes from female to male) with plasticity in the timing of its sex change, using a game theory model to describe the conditions under which an individual in the stock would change sex. An individual at a given size would change sex if its expected reproductive output at that size was greater as a male than as a female. This decision was made based on its current size and how it would fare in competition given the size of all other individuals in the stock. This model was embedded within a standard population model and varied parameters such as exploitation rate to determine the effects of fishing on these stocks. The results were compared to an otherwise identical, non-sex changing stock.
It was found that the non-sex changing stock under the same rate of exploitation as the hermaphroditic one had a slightly higher MSY in terms of numbers of individuals harvested but a lower biomass. This is due to the ability of the sex changing stock to replace the large individuals more rapidly. It was also shown that the sex changing stock may be able to maintain its sex ratio better than the non-sex changing one. However, under increasing exploitation, the sex changing stock maintains this ratio by changing sex at smaller sizes, decreasing the overall size of the average fish in the stock. The authors show that stocks of species with plasticity in the timing of sex change can be as resilient to fishing as non-sex changing stocks; however, overexploitation may lead to smaller individuals. The model presented in this study is flexible enough to include many of the different sex change cues, including those that are fixed, and could easily be included in stock assessments of hermaphroditic fish.
A black sea bass, used as the example in the study, being tagged. Photo: Orion Weldon