Southeast Ecological Science Center
Kirsten E. Luke1, Brice X. Semmens2, Phillippe G. Bush3, Bradley Johnson3 and Croy M. McCoy3
1Coastal Ecology & Conservation Research Group,
Presented at the 58th GCFI Conference San Andres Colombia November 2005
ABSTRACT: Infestations by the isopod Excorallana tricornis tricornis on Nassau grouper (Epinephelus striatus) caught in modified Antillean fish traps around Little Cayman Island were documented shortly after spawning season. Eleven of thirteen sexually mature (>50cm) Nassau grouper caught in traps during a 24 hours soak time were attacked and some mortality occurred. Divers reported seeing groupers under attack during the study period, suggesting that trapping did not exclusively precipitate attacks. Local fishermen reported that attacks were common immediately following Nassau grouper spawning season in mid-winter annually.
Crustacean zooplankton may influence the behavior and habitat preferences of at least some reef fish species through their dual roles as potential prey and parasites (Stepien & Brusca, 1985). While most species of the isopod Excorallana are considered scavengers (Guzman et al., 1988), E. tricornis tricornis (Hansen 1890) facultatively parasitize several genera of Caribbean marine fish (Delaney, 1984).
Nassau grouper, Epinephelus striatus (Bloch 1792) from Little Cayman, Cayman Islands (figure 1) caught in Antillean fish traps shortly after the winter spawning season were infested with parasites. Fish trapping was part of a Nassau grouper acoustic tagging project documenting the spatial and demographic influence of a large (~2000 fish; Whaylen et al., 2004) Nassau grouper spawning aggregation. Nassau grouper aggregate to spawn over a period of approximately 10 days during winter full moons. During the 2004-2005 spawning season, grouper on Little Cayman aggregated three times to spawn, on 26 January 2005, 23 February 2005, and 25 March 2005.
Table I. Catches from modified Antillean fish traps deployed for 1373 hours. Table lists each species caught, along with the number of individuals captured, the number of individuals with conspicuous and ongoing attacks by E. tricornis tricornis, and the number of fish found dead. Note that Nassau grouper are split into two rows, representing individuals larger than 50 cm TL (large enough to tag, and likely reproductively active) and fish smaller than 50cm TL (too small for tagging, and possibly not sexually mature).
If the fishermen are correct, the costs to this species associated with spawning may be of sufficient magnitude that a high degree of behavioral 'choosiness' regarding whether and when to release gametes might be expected. Moreover, recent observational studies suggest that fish on small aggregations (<100 fish) do not spawn (Gascoigne & Elliot, 2002). The conservation of Nassau grouper spawning aggregations is a matter of concern not only because aggregations are easily targeted by fishermen, but also because once aggregations have been fished to exhaustion (e.g. Florida Keys and Bermuda) the species has failed to recover, despite decades of subsequent protection (Cornish & Eklund, 2003). Parasite avoidance and behavioral infection-reducing mechanisms are one aspect of the evolving 'arms race' between parasites and hosts (Dawkins & Krebs, 1979; Barber et al., 2000). Host evolution maximizes the chance of reaching reproductive age, and, more generally, its overall individual lifetime reproductive potential (Poulin et al., 1994). Thus, behavioral parasite-avoidance mechanisms are likely regardless of current parasite loads; host behavior reflects the potential future costs of parasitism. If the energetic and physiological expense of reproduction exposes individual Nassau grouper to higher levels of parasitism (and ultimately higher mortality rates), the perceived reproductive benefits of gamete release on small aggregations with limited mate choice may well be outweighed by the lifetime fitness costs associated with spawning. Such behavioral 'decisions' are tantamount to an Allee effect (Allee, 1931), and could partially explain the lack of spawning by small aggregations and the failure of highly depressed stocks to recover.
The authors thank B. Luckhurst and T. Trott for field help, and P. Hillenbrand for logistical support. R. Brusca identified the isopod and provided formative ideas and guidance regarding this paper. Funding for the project was provided in part by the Department of Commerce (DOC), National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Conservation Fund [award# NA04NOS4630287] and the PADI Project AWARE Foundation. B. Semmens was supported by a Science To Achieve Results (STAR) United States Environmental Protection Agency (EPA) graduate research fellowship. The statements, findings, and conclusions in this report are those of the authors and do not necessarily reflect the views of DOC, NOAA, or EPA. Special thanks to the Reef Environmental Education Foundation (REEF) and REEF volunteers for facilitating this research.