E. Fishery-dependent Monitoring of Species that Aggregate to Spawn

Monitoring a fishery is critical for assessing its status over the long term, for determining appropriate management action and for evaluating the effects of management. Estimating catch is covered in Section VI. The current section explores how different types of catch data can be used to determine seasonality in fishing of aggregations and to identify trends in aggregation- focused fisheries over time. It also addresses important considerations for designing catch sampling programs.

Fishery monitoring must be done on a species by species basis. It is not valid to, say, monitor ‘groupers’ or ‘snappers’ as a general category that includes several or many species of grouper or snapper and hope thereby to understand the status of individual species. The reason for this is that ecological species interactions, especially among closely related species, mean that what happens to one species may influence what happens to another. As just one example, larger species of grouper tend to be more vulnerable, for a range of reasons, to most fisheries than smaller species for a given level of effort. In the Caribbean, as larger species declined, it is likely that competition with smaller species was reduced and this allowed an increase in biomass of smaller species. If only the category ‘groupers’ was to be recorded from the fishery, there might appear to be no overall changes in landings even when larger species were declining significantly because an increase in the smaller species would compensate.

It is possible to determine a marked fishing season, which might reflect aggregating behavior, by appropriately designed monitoring of the fishery. A distinct fishing season exists if the majority of landings of a particular species is taken, or the catch per unit of effort (CPUE) of landings are particularly high, at certain and consistent times of the year. Such seasonality might help in identifying an aggregation period, because during such periods catches or CPUE are often higher than at other times of the year. It is important, however, to recognize that catches can vary considerably throughout the year due to factors that might have nothing to do with aggregating behavior. Examples include weather conditions that strongly affect fishing activity, preventing or allowing access, for example, of boats to particular places at particular times. Trends in the fishery whereby fishers change their activities due to factors such as market demand or regulations elsewhere may also be important. A simple set of questions for fishery officers of fishers should help you to determine what factor(s) might be involved when regular changes in CPUE or landings are noted for particular species.

It is essential to assess the landings or, more appropriately, the CPUE at different times of the year if the fishery is to be effectively managed and the response of the fishery to fishing is to be evaluated over the long term (note that even if aggregations are exploited, the species could also be heavily exploited at non-aggregation times). For example, if a standardized program monitoring the fishery of a particular species shows that CPUE or landings decline over time (i.e., over a number of years) this may signal that over-fishing is occurring and that management might be needed. Note that CPUE is the value that is needed to obtain an idea of what is happening in the fishery if the fishing effort (such as number of boats or fishermen) is changing over time because changes in landings alone may not reflect anything about the species but reflect fisher behavior rather than fish numbers. In general, CPUE is preferred to landings data as an indicator of the status of a fishery and should be collected whenever possible (but see below the discussion on aggregation CPUE).

If the majority of the annual landings of a particular species occurs during its spawning (aggregation) period, then management attention might best be directed at the aggregation itself rather than on the species at other times of the year. If, on the other hand, declines in aggregating species occur and the species is fished all year round, then management of the aggregation may well not be sufficient to stop or reverse the decline. Aggregations are only part of the life history of aggregating species that may also be vulnerable to fishing at other times of their life cycle or moving to and from aggregations (Lindeman and Claro, 2003). It is important therefore to determine when and where, and how heavily, fisheries take place during the year to ensure appropriate management measures are developed for a given species in a given fishery.

Another important point to consider is that species that aggregate to spawn, and for which a large proportion of the landings is taken during the aggregation period, can be surprisingly difficult to monitor reliably based on aggregation landings alone, or even when CPUE is used. While total annual landings is the sum of aggregation and non-aggregation landings, landings or CPUE taken at the aggregation site may well not reflect the condition of the fished population over time. Given what we have already said about CPUE, why is this? The reason is that CPUE is considered to reflect abundance (in fisheries since we can not measure fish abundance directly we have to have some parameter that indicates abundance and CPUE is commonly used for this since it corrects for fishing effort). In species that aggregate, however, aggregation CPUE may stay high, even as population levels decline. This occurs if aggregation fishing does not remove most of the fish each during aggregation period but fish continue to aggregate even as population levels decline substantially. CPUE from the aggregation will, as a result, remain constant until numbers become severely reduced, and at some point CPUE will start to drop rapidly and the aggregation may cease to form (Sadovy and Domeier, unpublished manuscript).

What this suggests is that CPUE should, for exploited species that aggregate to spawn, be measured during the non-reproductive, as well as the reproductive season, as an indicator of fish abundance, even if most of the landings occur during the aggregation season. Alternatively, well- designed fishery independent surveys, involving estimates of fish numbers on aggregations would be recommended (see Section IV.B). Such considerations are critical for fully understanding trends in fished populations, are simple in concept and in most cases can effectively be built into current or new monitoring programs with careful planning.

In conclusion on fishery-dependent monitoring, it is essential, if a fishery is to be productive in the long-term, that monitoring programs be implemented. They do not have to be complicated. In fact, simple and clear monitoring programs are more likely to persist long-term than complex or expensive ones. Important decisions, however, do have to be made about when and how to monitor and which species to monitor. When and how have been discussed for aggregating species (i.e. CPUE should be measured outside of the aggregating season if at all possible and body size information can also give indications of long-term changes if carefully assessed – refer to Section IV.C). Which species are to be monitored will depend on monitoring capacity but, if this is limited, then decisions might consider tracking trends in particularly important species, or particularly vulnerable species. For example, many of the larger reef fishes are likely to be more vulnerable than smaller species if targeted specifically. Finally, care is also needed that the monitoring of similar-looking species is not affected by species misidentifications.