Parasitic Isopod Infestations on Post-Spawning
Nassau grouper (Epinephelus striatus) from Little Cayman, Cayman Islands

Kirsten E. Luke¹, Brice X. Semmens², Phillippe G. Bush³, Bradley Johnson³ and Croy M. McCoy³

 ¹Coastal Ecology & Conservation Research Group,
U.S. Geological Survey,
Center for Aquatic Resource Studies, 7920 NW 71st St., Gainesville, FL 32653
²Reef Environmental Education Foundation (REEF)
Pacific Coast Office, 4726 38th Ave NE, Seattle, WA 98105
³Department of Environment, Cayman Islands Government,
P.O. Box 486GT, Grand Cayman, Cayman Islands

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.

INTRODUCTION:

       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.
 

METHODS:

• April 12-21, 2005: Six Antillean traps were deployed at near shore sites around Little Cayman Island (figure 1 and 2)
• They were built from 5x10cm plasticized mesh around an oval frame (1.8m long, 1.2m wide, 0.6m tall)
• Traps were baited with squid, cut fish, conch, and lobster placed inside mesh bags and bait replenished every 2 days
• Traps were checked every 24h and then redeployed.
   

RESULTS:

• Each trap was set for approximately 229 hours, totaling 1373 trapping hours
• 51 fish, including 21 Nassau grouper (Table I) were caught
• Half of the Nassau grouper were infested with isopods (Figures 3-6); two were found dead in the traps.
• Larger (>50cm TL) Nassau grouper were more likely to be infested than smaller (<50cm TL) individuals (Table I; χ², d.f. = 1, P = 0.003).
• Infested fish were apparently physically and behaviorally affected by the infestation.
• Most exhibited:
• patches of skin discoloration (figure 4)
• cloudy eyes (figure 5)
• Lethargy and disorientation
• Infested individuals were sluggish when approached by divers and, when released, swam only a short distance before settling on the substrate. One individual released from a trap at 30m swam to the surface.
• Isopods were collected from 2 different grouper on subsequent days, and preserved in 95% ethanol for identification.
• All preserved isopod individuals were identified as E. tricornis tricornis [Figure 3(a,b,c)].
• 81 grouper were caught on hook and line for tagging during the January 2005 spawning aggregation; none was infested with E. tricornis tricornis.
• After consultation:
• Local fishermen reported that they have never targeted Nassau grouper immediately following spawning because of the parasites and their flesh is "soft and unmarketable"
• Local dive professionals reported seeing some Nassau grouper with conspicuous and heavy isopod infestations that were "obviously unhealthy"
  
  

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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).

* This individual was found dead in trap with the tail and rear portion of the body bitten off. The authors suspect a shark attacked the fish while its tail was through the trap mesh.

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DISCUSSION:

       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.

CONCLUSIONS:

• Stepien & Brusca (1985) documented nocturnal attacks on caged temperate marine fish by isopods and concluded that the attacks occurred because confinement restricted normal escape mechanisms.
• Traps are likely to increase the likelihood of infestation but are not the exclusive cause.
• Traps would certainly impede normal modes of escape; fish would be unable to visit cleaning stations.
• Post-spawning, larger grouper were more heavily infested and generally more unhealthy than pre-spawning, smaller grouper.
• These results and observations suggest that the cost of spawning (migrating, aggregating, producing and releasing gametes) greatly impacts the physical condition of the fish and their susceptibility to parasites.

References

• Allee, W. C. (1931). Animal Aggregations. A Study in General Sociology. University of Chicago Press: Chicago.
• Barber, I., Hoare, D. & Krause, J. (2000). The effects of parasites on fish behavior: an evolutionary perspective and review. Reviews of Fish Biology and Fisheries 10, 131-165.
• Dawkins, R. & Krebs, J. R. (1979). Arms races between and within species. Proceedings of the Royal Society of London B 205, 489-511.
• Delaney, P. M. (1984). Isopods of the genus Excorallana Stebbing, 1904 from the Gulf of California, Mexico (Crustacea, Isopoda, Corallanidae). Bulletin of Marine Science 34, 1-20.
• Guzman, H. M., Obando, V. L., Brusca, R. M. & Delaney, P. M. (1988). Aspects of population biology of the marine isopod Excorallana tricornis occidentalis Richardson, 1905 (Crustacea: Isopoda: Corallanidae) at Cano Island, Pacific Costa Rica. Bulletin of Marine Science 43, 77-87.
• Poulin, R., Brodeur, J. & Moore, J. (1994). Parasite manipulation of host behavior - should hosts always lose? Oikos 70, 479-484.
• Stepien, C. A. & Brusca, R. M. (1985). Nocturnal attacks on nearshore fishes in southern California by crustacean zooplankton. Marine Ecology Progress Series 25, 91-105.
• Whaylen, L., Pattengill-Semmens, C. V., Semmens, B. X., Bush, P. G. & Boardman, M. R. (2004). Observations of a Nassau grouper (Epinephelus striatus) spawning aggregation site in Little Cayman, including multi-species spawning information. Environmental Biology of Fishes 70, 305-313.
• Cornish, A. & Eklund, A. M. (2003). Epinephelus striatus. In: IUCN 2004. 2004 IUCN Red List of Threatened Species. http://www.redlist.org Date of Download 09 April 2005.
• Gascoigne, J. & Elliot, M. (2002). Nassau grouper and queen conch in the Bahamas: status and management options. Bahamas Reef Environmental Education Foundation (BREEF) Website. http://www.breef.org/grouperfull.pdf Date of Download 05 May 2005.

Acknowledgments:

       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.