Management of fish species that aggregate to spawn

The appropriate management of fish species that aggregate to spawn will depend on a number of factors. From a biological perspective, an important question that needs to be asked is whether fishing activity on, or other human disruptions of, a spawning aggregation, could have a negative impact on reproductive behaviour or egg output, and, therefore, whether aggregations themselves need to be protected. There are a number of reasons for asking whether aggregations need protection in addition to, or perhaps instead of, other forms of management. In this newsletter, I only address some biological perspectives: practical, social and other considerations will be examined later. Note that this column does not necessarily reflect the opinion of us at SCRFA; rather it is intended to stimulate discussion, provoke thought, and, hopefully, encourage feedback.

There are several possible ways in which fishes gathered for spawning could possibly be directly or indirectly affected by human activities. These include physical disturbance of spawning or courting fish by fishing activity, or impacts on social systems by selective removals of fish by sex, size or genotype. It is also possible that the presence of divers during research, acoustic impacts or tourism activities might affect aggregated fish. Whether such effects do occur as a result of aggregation-fishing (as opposed to from fishing activity outside of the aggregation period) or other activities, and whether they affect reproductive output of the population in the short or long term is largely unknown. However, it is of interest to look at the general fish literature (i.e. not just reef fishes) for indications of possible impacts, as pressures to fish or otherwise focus on aggregations increase.

Stress in fishes can affect their reproduction. There is some evidence in Atlantic cod (Gadus morhua) that the physical act of trawling through aggregated fishes disturbs reproductive activity. Cod have quite complex reproductive behaviours (e.g., Hutchings et al., 1999) and the physical passage of trawl nets can disrupt these for extended periods of time in the field (Morgan et al., 1997). A laboratory study on the potential effects of stress, such as due to trawl avoidance, showed that courtship and other behaviours were reduced by disturbance, but that this did not appear to affect egg production, fertilization rates or hatching success. On the other hand, stressed fish produced abnormal larvae more frequently (Morgan et al., 1999).

In other species, including common snook (Centropomus undecimalis), pink snapper (Pagrus auratus) and red gurnard (Chelidonichthyes kumu), stress in captivity is known to cause changes in hormone levels, fecundity, egg size and development, and egg survival (examples cited in Morgan et al., 1999). Snook and pink snapper (a sparid) both aggregate to spawn: a recent histological study on common snook showed that females caught using hook and line from a spawning aggregation did not interrupt or finish spawning, or exhibit egg breakdown. While there are indications that stress can affect reproduction, the extent to which fishes might be stressed in exploited or disturbed spawning aggregations, is not known.

Disruption of spawning or courting behaviour by fishing (removals of fish), due to the presence of divers, or from acoustic impacts, we likewise know little about. It may be that nothing ultimately inhibits the biological imperative to spawn; that the fish just wait until intrusions go away. Lobel and Neudecker (1985), for example, found that a hamlet (pair-spawner), disturbed by divers, did not abort spawning but did change their behaviour, continuing clasps later into the night or moving closer to the substrate. Nonetheless, we do not understand what occurs in other species or in aggregated fishes when disturbed. We know that the social structure within aggregations can be quite specific, that males and females may have different patterns of arrival and departure, and that there is intense courtship, and, presumably, mate selection. I am not aware of any literature on the possible impacts of the presence of divers or fishers on social and mating behaviours in such cases, but this is one area that merits scrutiny. Field biologists suspect that some species respond to divers and others much less so; they also know that the presence of lights can interrupt or possibly prevent spawning. The possible acoustic impacts on fish spawning aggregations are largely unknown; one study on a croaker suggested little impact on drumming behaviour among assembled fish, while other studies on fish have shown negative effects on behaviour (Jeremy Hall Memorial University Newfoundland) (http://web.mit.edu/seagrant/aqua/cfer/acoustics/exsum/collins/extended.html).

A number of aggregating species are known to change sex, among these both protogynous (female to male) and protandrous (male to female) sex-changers. For all such species studied, sex change appears to be controlled by aspects of the social environment or mating group ranking rather than by absolute size or age, or genotype. For many species, their spawning aggregations are the only times that large numbers of conspecifics are likely to come together. For sexchangers, therefore, important cues relevant to sex ratios and sex change may be communicated only at this time (e.g., Shapiro et al., 1993). If larger (mostly male) fish typically live deeper than smaller (more likely to be female) fish, or if the sexes are widely dispersed for most of the year, as they probably are for many larger reef species, then they may only be able to assess adult sex ratios in the population while gathering to spawn. This could have implications for subsequent sex change schedules. Fishing on aggregations could, conceivably, disrupt information availability or its transfer relevant for sex change.

In conclusion, biological studies and theory suggest that certain biological attributes of fishes could predispose some species to negative impacts on reproductive of fishes could predispose some species to negative impacts on reproductive outputs from aggregation-fishing or other disturbances, either directly or indirectly. What little evidence there is, however, is inconclusive and focused studies are needed to test hypotheses of possible impacts. What is clear is that a precautionary approach is needed. For example, in the case of activities such as dive tourism, precautionary guidelines should be developed that include no lights, divers remaining in one place and not approaching aggregated fish too closely.

Refer to our SCRFA Methods Manual, Colin et al., 2003, for a brief discussion on diver disturbance and how to reduce it.

Lobel, P.S., and S. Neudecker. (1985). Diurnal periodicity of spawning activity by the hamlet fish, Hypoplectrus guttavarius (Serranidae). IN: The Ecology of Coral Reefs, Symposia Series for Undersea Research. M.L. Reaka (ed.), 71-86. Vol. 3. Rockville, MD: NOAA, National Undersea Research Program.
Lowerre-Barbieri, S. K., F. E. Vose and J. A. Whittington. (2003). Catch-and-release fishing on a spawning aggregation of common snook: does it affect reproductive output? Transactions of the American Fisheries Society 132:940-952
Hutchings, J. A., T. D. Bishop and C. R. McGregor-Shaw. (1999). Spawning behaviour of Atlantic cod, Gadus morhua: evidence of mate competition and mate choice in a broadcast spawner. Canadian Journal of Fisheries and Aquatic Sciences 56:97-104
Morgan, M. J., E. M. DeBlois and G. A. Rose (1997). An observation on the reaction of Atlantic cod (Gadus morhua) in a spawning shoal to bottom trawling. Canadian Journal of Fisheries and Aquatic Sciences 54 (Suppl. 1):217-223
Morgan, M. J., C. E. Wilson and L. W. Crim. (1999). The effect of stress on reproduction in Atlantic cod. Journal of Fish Biology 54:477-488
Shapiro, D. Y., Y. Sadovy and A. McGehee. (1993). Size, composition, and spatial structure of the annual spawning aggregation of the red hind, Epinephelus guttatus (Pisces: Serranidae). Copeia 1993: 367-374

Aug 2006