Reproduction of reef fishery species

Summary

Although reproductive patterns in tropical fishery species are characterized by diversity, some interesting trends are clearly emerging, despite the few families studied in detail. Reproductive output is highly variable, both within and among species, years and individuals. Especially in larger species, the differential between the fecundities of different-sized conspecifics may be orders of magnitude. Environmental and biological factors influence when and where reproduction takes place, with annual spawning seasons ranging from as little as a week or two, to much of the year; the notion that tropical marine species are all characterized by protracted spawning is unfounded. Most species produce pelagic eggs and, characteristic of reef fishes in general, all produce pelagic larvae. Females may spawn a few times to many times annually. Spawning often takes place towards dusk, in some cases at specific times in the lunar or tidal cycle. Evidence for lunar cyclicity in most larger species, however, is not strong except among a number of species which aggregate to spawn. Spawning occurs in areas of residence or at well-defined aggregation sites, metres or many kilometres away from home sites. Males and females mate either pairwise or in small groups which characteristically comprise one female and several males; the size of the testis relative to the body (gonadosomatic index) of ripe males apparently accords with mating pattern - larger for group and smaller for pair spawners. Larger, more mobile, species within a family tend to migrate to reproduce in aggregations which are frequently, but by no means exclusively, located offshore and close to deep waters. Such sites may be quite distinct but there is no evidence to indicate that they are unique. It is not clear whether aggregation spawning and the preference for specific spawning locations evolved largely for the benefit of larval dispersal or survival, to enable males and females that live somewhat dispersed to come together or for some other reason.

Morphological factors such as body form and size are also important determinants of reproductive output, with more compressed forms exhibiting lower relative fecundity at length than more rounded body forms. These factors influencing egg output per spawn, combined with maturation size and spawning frequency and duration, are all pertinent to our understanding of reproductive patterns and strategies and inter- and intraannual variation in reproductive output. By elucidating the variables that determine egg production we might better understand relationships between numbers and activities of spawning adults and the resulting temporal, spatial and abundance patterns of larval and juvenile appearance on reefs.

From a fishery perspective, there is an urgent need to better estimate and maintain reproductive output in exploited populations. For example, spawning aggregations, because they are often consistent in time and space, may be heavily targeted by fishers. Of particular concern is evidence that aggregations can be decimated by heavy fishing with a possible concomitant loss of larger individuals or disruption of reproductive activity. Because the effects of intense aggregation-fishing, in both the short and long term, are largely unknown but may seriously compromise reproductive output, they should be accorded particular attention for research and management. Similarly, quite a few commercially exploited reef families are hermaphroditic. The impact of fishing or conventional management practices on hermaphroditic species is not understood and will be difficult to assess until we come to understand, among other things, the factors that induce sexual changeover. Research and monitoring programmes designed specifically to address these issues provide a rich and exciting challenge to the curious ecologist and the innovative fishery biologist.