C. Monitoring Spawning Success

Monitoring the spawning success (i.e., number of eggs produced) of reef fishes is not easy. Monitoring numbers of fish in aggregations is largely intended to allow the determination of whether populations of fish in aggregations are stable, declining or increasing, but seldom deal with the actual number of eggs being produced by an aggregation. A number of suggested methods for monitoring the numbers of fish at an aggregation site have been included in Section IV.C. The methods for determining the numbers of eggs being produced by spawning fishes are even more tenuous and difficult than counting numbers of aggregated fish. The number of spawning events in a given area would be one way to potentially compare spawning success at different times, but we would have to make the assumption that roughly equal numbers of eggs are produced from each spawning. However, this will not be true if there are marked differences in female size (body size is related to fecundity), while we already know that there can be much variation in the number of eggs released per spawning event (or egg batch) even by individual females. The numbers of individual fish spawning would also be a useful way to look at possible spawning success, but again doubts arise concerning the actual number of propagules produced. Finally some method of monitoring the eggs produced within an aggregation directly would be very useful, such as the "moored plankton nets" described in Section V.A. Moored nets would likely produce similar results at different times, differences in egg numbers collected reflecting differences in spawning activity, if other factors such as currents, wave action and weather remained constant. Of course, they don't and may well change greatly the number of eggs collected versus those spawned.

We would suggest that a variety of approaches would have the best chance of monitoring spawning success and detecting changes over time. Numbers of spanners, number of spawns and number of eggs captured can be assessed independently, and hopefully the trends among the three are similar, giving some confidence. Where there are differences among the three measures and perhaps among data collected at different times (months, years), the reasons for the differences might be assessed.

In all cases where a given aggregation site is intended for long-term monitoring, be certain reports written about the present status of sites include adequate information for someone to locate and repeat surveys of the site in future years. While it has been emphasized before, this fact should not be forgotten for any aspect of aggregation-related studies, and should be a guiding principle in the gathering of information.

It may some day be feasible to attempt to estimate spawning success by the number of fish recruiting from the plankton to benthic habitats. However in this regard our knowledge is so rudimentary that to make an assumption without reliable data on spawning activity, transport of eggs and larvae, and recruitment levels would be reckless. Indeed, for most fishes, the relationship between the number of spawners and the number of recruits is unknown and, given the substantial mortality that must occur in the planktonic phase, may never be clearly estimated. Nonetheless, the relationship between spawning as a source of eggs and recruitment as the ‘sink’

Figure 53. (Left) Typical satellite tracked current drifter, with 10 m long drogue (“holey sock”) with 5 m line to surface float that contains transmitter and batteries (PLC). (Right) Tracks of current drifters released at or near Nassau grouper spawning sites (after Colin, 1996).

is at the core of the efforts to design and best place marine protected areas to provide for downstream seeding of recruits. Ideally studies should attempt to gather data on all of the factors related to these questions, so that we can begin to understand the relationships between spawning and recruitment for those fishes with aggregation spawning. Most work on reproductive success has utilized demersal fishes, particularly damselfishes (Pomacentridae), and while their early life history has many similarities to planktonic aggregation spawners, there may be major differences.

Monitoring recruitment can often provide information regarding the timing of spawning through back-dating to the time of spawning using otolith daily increments in juvenile fishes (Colin et al., 1997). In species for which the spawning period is not known, this might provide clues as to when to look for aggregations or spawning in the field. It can also help if a large number of juveniles are sampled and aged to determine the range of the spawning period and lunar/seasonal periodicity. Whenever working back from otolith ages, it must remembered that the fish captured and aged may not reflect the population as a whole, having been only those individuals which survived their planktonic life and happened to be captured for study. Methods for capturing juveniles for otolith work include light traps, plankton trawls for advanced state pelagic juveniles and channel nets. Channel nets, for example, have been used for monitoring recruitment of groupers (Keener et al., 1988, Colin et al., 1997, Shenker et al., 1993). Both light traps and channel nets can capture large pelagic juveniles alive that can then be used for other experimental work, if desired (Doherty, 1987).