View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Aquatic Commons Caribbean Journal of Science, Vol. 45, No. 2-3, 282-303, 2009 Copyright 2009 College of Arts and Sciences University of Puerto Rico, Mayagüez Nocturnal fish movement and trophic flow across habitat boundaries in a coral reef ecosystem (SW Puerto Rico) Randall D. Clark 1 * , Simon Pittman1 , Chris Caldow 1 , John Christensen1 , Bryant Roque 2 , Richard S. Appeldoorn 2 , and Mark E. Monaco 1 1 National Oceanic and Atmospheric Administration, Biogeography Branch, 1305 East-West Highway, Silver Spring, MD 20910 USA 2 Department of Marine Sciences, University of Puerto Rico, Mayaguez, Puerto Rico 00681-9013 USA * Email: [email protected] A BSTRACT. — Few studies have quantified the extent of nocturnal cross-habitat movements for fish, or the influence of habitat adjacencies on nutrient flows and trophodynamics. To investigate the patterns of noc- turnal cross-boundary movements of fish and quantify trophic connectivity, fish were sampled at night with gillnets set along the boundaries between dominant habitat types (coral reef/seagrass and mangrove/ seagrass) in southwestern Puerto Rico. Fish movement across adjacent boundary patches were equivalent at both coral reefs and mangroves. Prey biomass transfer was greater from seagrass to coral reefs (0.016 kg/km) and from mangroves to seagrass (0.006 kg/km) but not statistically significant, indicating a balance of flow between adjacent habitats. Pelagic species (jacks, sharks, rays) accounted for 37% of prey biomass transport at coral reef/seagrass and 46% at mangrove/seagrass while grunts and snappers accounted for 7% and 15%, respectively. This study indicated that coral reefs and mangroves serve as a feeding area for a wide range of multi-habitat fish species. Crabs were the most frequent prey item in fish leaving coral reefs while molluscs were observed slightly more frequently than crabs in fish entering coral reefs. For most prey types, biomass exported from mangroves was greater than biomass imported. The information on direction of fish movement together with analysis of prey data provided strong evidence of ecological linkages between distinct adjacent habitat types and highlighted the need for greater inclusion of a mosaic of multiple habitats when attempting to understand ecosystem function including the spatial transfer of energy across the seascape. K EYWORDS. — connectivity , habitat boundaries , coral reef ecosystems , mangroves , fish prey , nocturnal fish movement , seascape Introduction seagrass and sand beds to forage at night Many fish species in tropical coral reef before returning to the more structured hab- ecosystems connect multiple habitats itat types by dawn (Ogden and Ehrlich 1977, through regular nocturnal migrations into Helfman et al. 1982; Rooker and Dennis neighboring habitats, with some species 1991; Nagelkerken et al. 2000; Monaco et al. using several distinct resources across a 2009). Seagrass beds provide a high abun- compositionally complex mosaic of habi- dance of food and suitable refuge in low tats (Parrish 1989; Kramer and Chapman light conditions, thus functioning as a com- 1999; Nagelkerken et al. 2000; Cocheret de plementary or supplementary resource for la Moriniere 2002; Dorenbosch et al. 2005; many multi-habitat species (Pittman et al. Unsworth et al. 2008). For instance, Meyer 2004, Pittman et al. 2007). This pattern of et al. (1983) reported at least 15 fish fami- day-night resource use has likely evolved to lies that leave coral reefs to forage in neigh- maximize growth while minimizing mor- boring areas. Haemulidae (grunts) and tality through predation (Dahlgren and Lutjanidae (snappers) in the Caribbean Eggleston 2000, Grol et al. 2008). Community have frequently been observed to under- studies have shown that nocturnal excur- take sun-synchronous migrations by leav- sions can result in pronounced diel shifts in ing their daytime shelter on coral reefs and fish assemblage composition across inter- mangroves at dusk to migrate to adjacent connected coral reef ecosystems (Kopp et al. 282 NOCTURNAL CORAL REEF FISH MOVEMENT, SW PUERTO RICO 283 2007; Unsworth et al. 2007). Since most stud- is an urgent need for quantitative infor- ies of fish movement occur during daylight mation capable of identifying pathways of hours, little is known about the identity and energy flow and determining the influence abundance of nocturnal trans-boundary of habitat boundaries and habitat adjacen- movements. Earlier studies of migrating cies on these processes. In this way, we begin resident species (e.g. grunts, snappers) sug- to link ecological patterns with dynamic gest net trophic flow of prey biomass to be ecological processes across structural mosa- greater coming back to the resting habitat ics of habitat in coral reef ecosystems. This type (Ogden and Ehrlich 1977; Ogden and fundamental ecological information can be Zieman 1977; McFarland et al. 1979). applied to help understand factors includ- Fish migrations have long been consid- ing human modifications that may enhance ered to be important conduits of organic or limit energy flow across coral reef eco- and inorganic material to and from coral systems and may also help determine the reefs, mangroves and surrounding areas optimal design of Marine Protected Areas (Birkeland 1985; Meyer and Schultz 1985; (MPAs) and help refine designations of Parrish 1989; Sheaves 2005), yet since the Essential Fish Habitat (EFH) (Polunin and publication of Randall’s (1967) dietary sur- Roberts 1993; Murray et al. 1999). vey of 212 Caribbean fish relatively little This paper examines inter-habitat trophic quantitative information is available on connectivity across the seascape of La multi-species foraging and diets. Even fewer Parguera, SW Puerto Rico by examining the studies have focused on the exchanges flow of fish biomass and their associated of organisms and dietary material across prey. Numerous adult and sub-adult fish boundaries between neighboring habitats. species were captured during their noctur- This is an important knowledge gap since nal excursions across: (A) the boundary of fishes have been shown to be a significant coral reefs and seagrasses (Cr-Sg ), and (B) source of organic carbon and other nutrients the boundary of mangroves and seagrasses in tropical marine ecosystems (Bray et al. ( Mg-Sg ) and quantitative data on their prey 1981; Ogden and Gladfelter 1983). For exam- consumption were collected. Trophic flow, ple, early experiments on the effects of excre- defined here as the cross-boundary move- tion and defecation from migrating schools ment of fish and prey biomass, was exam- of fish over coral reefs indicated significant ined at Cr-Sg and Mg-Sg boundaries with inputs of ammonium and phosphorous that special emphasis on the role of Haemulidae may enhance the growth of macroalgae (grunts) and Lutjanidae (snappers) due to (Meyer et al. 1983; Meyer and Schultz 1985). their abundance in coral reef ecosystems In addition, foraging fish have been identi- and well documented nocturnal migrations. fied as a key redistributer of sediment par- Three main questions were addressed ticles in coral reef ecosystems (Alheit 1981) through quantitative descriptions and hypo- and the transport of nutrients and transfer th esis testing: of energy away from mangroves by mobile animals can have important consequences (1) What are the quantities of fish and the for recycling in mangroves (Sheaves and biomass of their consumed prey that Molony 2000; Sheaves 2005). are moving into and out from coral Parrish (1989) and Polis et al. (1997) reefs and mangroves through noctur- argue that trophic interactions that con- nal excursions? nect discrete habitat types can exert a major (2) Are haemulids (grunts) and lutjanids influence on the local abundance and distri- (snappers) the primary conduits of bution of organisms through both “bottom- fish biomass and nutrients into and up” processes as a result of cross-habitat out from coral reefs and mangroves transport of materials and nutrients and through nocturnal excursions? top-down processes as a result of preda- (3) What are the dominant prey items tion. As we progress toward understanding being transported into and out from seascape structure and the dynamics and coral reefs and mangroves through energy pathways across the seascape, there nocturnal excursions? 284 R. D. CLARK, ET AL. METHODS 3-20 m at Cr-Sg sites and 1-3 m at Mg-Sg sites. Study Area Gillnet sampling was conducted dur- Sampling fish ing nine surveys from June 2000-December 2002 across the insular shelf off La Parguera, A digital benthic habitat map (Kendall Puerto Rico ( Figure 1 ). Survey missions et al. 2001) displaying only the major hab- were conducted every three to four months. itat types of coral reefs, seagrasses and The shoreline and islands of the area are mangroves was used to randomly select hab- lined with mangrove communities domi- itat boundaries for sampling within the La nated by the red mangrove Rhizophora man- Parguera coral reef ecosystem. The habitat gle . Adjacent sediments support seagrasses map was used to stratify the study area into (dominated by Thalassia testudinum ), mac- two unique strata: (1) coral reef/seagrass roalgae and unvegetated sand and sandy ( Cr-Sg ) and (2) mangrove/seagrass ( Mg-Sg ). mud interspersed with coral reefs and During each survey five to eight gillnets patch reefs, which vary in habitat size and were set at each boundary. Gillnets were benthic community composition. The tidal 100 m long and had 5 x 5 cm nylon mesh size range was relatively small (<0.5 m), and and were deployed along the Cr-Sg habitat mangrove prop roots at the seaward edge boundary running parallel to the reef edge of the mangrove stands were continually and along the Mg-Sg habitat boundary run- immersed throughout the tidal cycle. Water ning parallel to the mangrove edge. Nets depths for sampled areas ranged from were set by boat at dusk and retrieved at Fig. 1. Location of study area, dominant benthic habitat types using NOAA’s benthic habitat map (Kendall et al., 2002) and gillnet sampling sites.