Allan W. Stoner, Ph.D., Martha H. Davis, M.S., Catherine J. Booker, M.S.
Report produced by Community Conch for: Bahamas Department of Marine Resources, Bahamas National Trust
PROJECT SUMMARY
This field study was designed for two primary objectives – to provide baseline data on the distribution and abundance of queen conch in a proposed marine protected area (MPA) in the southeast Berry Islands, and for a stock assessment of queen conch in the historically important fishing grounds on the Berry Islands bank. The study was conducted during June and July 2009, with a combination of SCUBA and towed diver surveys at more than 300 locations. On average, an area of more than 5,000 m2 (0.5 hectare) was surveyed for conch at each location.
Adult queen conch on the Berry Islands bank were most abundant in a depth zone 5–10 m deep in an area west of Rum Cay on the southern edge of the bank. Subadult conch (i.e., 3 yr old 'rollers') were most abundant in the same general location but in depths less than 2.5 m. Average densities in this south sector were 118 adults per hectare and 70 subadults per hectare. North and west sectors of the fishing ground had extremely low conch densities, typically less than 4 adults per hectare, and 8 subadults per hectare. Total numbers of conch for the survey area were approximately 2.54 million adults and 1.61 million subadults. The southern sector of the bank contained more than 98% of the adults in the total surveyed area and 96% of the subadults.
In the MPA, queen conch were surveyed on the shallow bank north and west of the islands. In this area, densities averaged just 4.4 adults per hectare and 13 subadults per hectare. On the deeper island shelf south and east of the islands average densities for adults ranged 3.7 to 27.4 per hectare, with highest density in a depth range of 5-10 m. Subadult densites were less than 3 per hectare at all depth intervals. A total of just 9,200 adults and 39,400 subadults were estimated for the shallow bank area encompassed by the proposed MPA.
Juvenile aggregations (>50 individuals/ha) were observed at just 10 locations, five in the proposed MPA and one just outside the northwest corner of the MPA. The other four sites occurred west of Chub Cay, along the southern edge of the bank. Two nursery grounds near Cat and Vigilant Cays studied by Iversen in the 1980s now have very few conch.
The surveys were conducted during peak mating season for queen conch and offered insight into the status of the reproductive population on the Berry Islands bank. Mating behavior was observed at only 15 of 308 survey locations, with just one mating pair observed in the MPA. The number of mating pairs was directly proportional to conch density, with highest numbers found in an area along the southern bank edge characterized by small phenotype adults (average shell length = 15 cm). These small adults probably have low fecundity. Outside the area of highest adult density, the adults on the Berry Islands bank had thin shell lip thickness and were probably not yet reproductively mature. Even in the high density areas, mating frequencies were far below those predicted by earlier studies of density-dependent mating in the Exuma Cays Land and Sea Park. Logistic regression showed that a 50% probability of mating occurred at 335 adults per hectare and 90% probability at 500 adults per hectare. Very few locations had these densities.
Given the low density of queen conch adults over most of the Berry Islands bank fishing grounds, relative youth of the adult population except in the area west of Rum Cay where adults were very small, low mating frequency, and apparent loss of historically significant juvenile populations, it seems likely that recruitment overfishing is occurring. Management actions need to be taken to preserve the reproductive potential, protect juvenile aggregations, and control fishing mortality. Suggested possible actions include developing and implementing a conch management plan specific to the Berry Islands, expanding or moving the MPA boundaries, prohibiting collecting conch with hookah gear, requiring that conch be landed in the shell to reduce illegal harvest of underage conch, and instituting a closed season for conch fishing during the reproductive season. Informal interviews with Berry Islanders suggest that they recognize the declining fishery for queen conch and favor management actions such as those listed. The success of a fisheries management plan will likely depend upon community involvement in management decisions, and fisheries monitoring and enforcement.