Relative predation risk for fishes along a subtropical mangrove–seagrass ecotone

Abstract

Many fishes shelter in mangrove habitats by day and forage mostly in seagrass beds by night. This pattern of diel habitat use has been attributed to a predator avoidance strategy, whereby predation risk is reduced by alternating between the cover afforded by prop-roots during the day and darkness at night. We employed a series of diel tethering experiments in Biscayne Bay (Florida, USA) to empirically examine whether relative predation pressure on fishes is lower at night than during the day and to compare relative predation pressure on fishes at different distances from the mangroveseagrass ecotone. Pinfish Lagodon rhomboides ranging from 10 to 17 cm in total length were tethered during day and night at 10, 50, and 110 m from the mangrove-seagrass ecotone. Pinfish removal rates at night were twice as high as during the day, which contradicts the idea that darkness provides 'cover' during nocturnal foraging in seagrass. Predation losses were highest nearest the mangrove edge and decreased with increasing distance from shore. Our results agree with those of other tethering studies that marine ecotones, or transition zones between refuges and feeding sites, can be areas of high predation pressure for fishes. KEY WORDS: Predators · Edge effects · Proximity gradient · Shark · Snapper · Porgy · Grunt Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 401: [259][260][261][262][263][264][265][266][267] 2010 tor access (Shulman 1985, Blaber 2007, Rypel et al. 2007). Most previous studies, which have been primarily diurnal, have consistently reported fish densities to be higher nearest the mangroves compared to offshore (e.g. Jelbart et al. 2007, Newman et al. 2007, Unsworth et al. 2008. However, direct field-based evidence evaluating how predation risk varies spatially in seagrass beds in relation to mangrove proximity is lacking.Ecotones, or habitat transition zones, are recognized as hunting corridors for predators in a variety of systems (Decamps & Naiman 1988, Ries & Sisk 2004. Organisms are likely at highest risk from predation when crossing ecotones between sheltered and feeding patches, due to the high concentration of individuals in refuges and predictability of their foraging movements. For example, off the coast of South Africa, hunting white sharks (Carcharodon carcharias) patrol fur seal (Arctocephalus pusillus pusillus) refuge entry and exit points in attempts to ambush seals as they leave for, and return from, foraging (Martin et al. 2005, Hammerschlag et al. 2006, Martin et al. 2009). Although previous studies in a variety of habitats (e.g. reefs, Shulman 1985) have reported predation pressure to be high near ecotones, or habitat edges, this has not been previously reported in mangroveseagrass habitats and results of most published diurnal fish abundance surveys indicate highest fish densities nearest the mangroves (Jelbart et al. 2007, Newman et al. 2007, Unsworth et al. 2008.Recently, Hammerschlag & Serafy (in press) examined nocturnal abundance ...

Intrapopulation variation in habitat use by two abundant coastal fish species

“…In back reef systems, seascape attributes have been shown to be a primary determinant of habitat use of fishes (Pittman et al 2007). Many coastal fishes, including snapper, generally prefer mangroves surrounded by dense seagrass (Pittman et al 2007) since mangrove prop roots provide shelter (Hammerschlag et al 2010) and seagrass beds contain high densities of prey items for coastal fishes (Orth et al 1984). In the present study system, potential resource pools, such as seagrass, were patchily distributed across the creek system.…”

Influence of predation risk and food supply on nocturnal fish foraging distributions along a mangrove–seagrass ecotone

“…To seasonally correspond with the period of seine sampling from July to September 2008 (wet season), we assessed nocturnal patterns of predator encounter rates along our transects using tethering experiments (Hammerschlag et al 2010a). Pinfish Lagodon rhomboides were used as bait because individuals in the size range corresponding with the focal fishes were readily available from commercial suppliers and pilot studies demonstrated that pinfish were both physically and behaviorally robust to our tethering process (Hammerschlag et al 2010a). Specifically, during pilot work, no fish died, exhibited signs of injury, or broke from their tether lines.…”

“…Finally, to secure the tether to the center line, it was connected to the center line using a snap-swivel (0.5 g). This snap-swivel attachment permitted pinfish to move freely in a vertical cylinder, with a 2 m diameter, from the seagrass to the water surface (Hammerschlag et al 2010a).…”

“…In cases where fish did not match an IFD, we investigated whether predation risk might be responsible for deviations from model predictions by comparing tether predation losses at each distance from shore using logistic regression on data presented by Hammerschlag et al (2010a). Predation rate-distance relationships did not differ significantly among transects (ANCOVA, p > 0.40), thus a single relationship between predation rate and distance from shore was calculated following Baker & Sheaves (2007) and applied along the entire distance gradient for all 3 focal fishes.…”

“…Nocturnal selection of seagrass beds by fishes as primary feeding sites appears to be related to elevated availability and accessibility of prey, many of which emerge from the vegetated substrate at night (Mattila et al 1999, Nagelkerken et al 2000, Unsworth et al 2007, Valentine et al 2007). However, how predation risk and food supply influence fish distribution patterns within seagrass beds during foraging at night remains poorly understood (Hammerschlag et al 2010a). Recent evidence suggests that predation risk in these systems may be relatively high, especially at night (Chittaro et al 2005, Rypel et al 2007, Doren-bosch et al 2009) and thereby likely to influence fish distribution patterns (Dahlgren & Eggleston 2000, Grol et al 2008.…”