February 15, 2012

Reef Check Spotlight: Why is Diver Monitoring So Important to Manage Reef Fisheries?


By Reef Check California Director Dr. Jan Freiwald
Many species of fish gather together in one area to spawn and reproduce. Smart fishers can target these areas and times and reap a high catch rate. Unfortunately, this can lead to rapid over-exploitation of these fisheries due to the large number of mature (reproductive fish) removed from the population before they have a chance to reproduce. In addition, if only data from fish catch is used by managers — it is possible for a decline in population size to be hidden from the managers for some time. Therefore it is important for fisheries managers to have access to what is called “fisheries independent” data such as the monitoring results carried out by Reef Check divers. Reef Check data is fisheries independent because the actual number of fish on rocky reefs are counted — in comparison to “fisheries dependent” data such as total catch.

A recent study by Brad Erisman et al., (2011) documented this problem in two southern California fisheries- the barred sandbass (Paralabrax nebulifer) and the kelp bass (P. clahtratus). Both these species aggregate during spawning, and the commercial fisheries were closed in 1953 because of concerns of potential overfishing. But the annual catch from recreational fishing remained stable or increased over a 30 year period through the 1990s, apparently indicating no problems. In fact, the actual population sizes of these two species declined dramatically by about 80% during this period based on diver surveys of actual numbers of fish on reefs.


The fact that the catch remained the same for such a long time period is due to the fish being targeted at high density aggregations and therefore being caught in high numbers even if overall population density is declining. Since the majority of the annual catch of these species is landed during these spawning aggregations it creates the impression of a sustainable fishery. This effect is termed hyperstability, meaning that the fishery seems to be stable while in reality the populations are declining. The data based on the fishing effort and annual catch did not reflect the true signal of population decline. The authors state that fisheries dependent data “created the illusion that harvest levels for both species were sustainable and stock abundances were stable”. Based on this information, resource managers maintained the same catch levels and have not adjusted their management strategy because the true decline of the populations was hidden from their ‘view’.

This study demonstrates the importance of fisheries independent data collection to gain insights into the population dynamics of exploited species. Without diver surveys or other independent measures of population density or biomass, the decline of these two species in southern California would not have been detected. Reef Check is monitoring both of these species in southern California and is working with fishers in Baja to develop sustainable fisheries for other aggregating species, such as groupers found along the Baja peninsula. Unfortunately, many open-water fisheries are very difficult to directly monitor. The lack of fisheries independent data is one reason why 85% of the world’s fisheries are considered overfished or have collapsed.

Erisman, B.E., Allen, L.G., Claisse, J.T., Pondella, D.J., Miller, E.F., and Murray, J.H. 2011. The illusion of plenty: hyperstability masks collapses in two recreational fisheries that target fish spawning aggregations. Can. J. Fish. Aquat. Sci. 68(10): 1705-1716.