Importance of Mangrove Shorelines for Rainbow Parrotfish Scarus Guacamaia: Habitat Suitability Modeling in a Subtropical Bay
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Vol. 15: 87–98, 2012 AQUATIC BIOLOGY Published online April 24 doi: 10.3354/ab00412 Aquat Biol Importance of mangrove shorelines for rainbow parrotfish Scarus guacamaia: habitat suitability modeling in a subtropical bay Ethan G. P. Machemer1, John F. Walter III2, Joseph E. Serafy2,3, David W. Kerstetter1,* 1Nova Southeastern University Oceanographic Center, 8000 North Ocean Drive, Dania Beach, Florida 33004, USA 2NOAA-NMFS Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, Florida 33149, USA 3University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA ABSTRACT: Rainbow parrotfish Scarus guacamaia is a coral-reef herbivore that requires both mangrove and coral-reef habitat to complete its life cycle. This species is listed as ‘vulnerable’ by the International Union for Conservation of Nature. The present study used a long-term visual sur- vey of mangrove fishes to develop a predictive habitat model for juvenile S. guacamaia. The fac- tors tested were temperature, dissolved oxygen, salinity, average depth, distance from offshore channel openings, temperature variation (ΔT), and salinity variation (ΔS). The average depth, dis- tance from offshore channel openings, ΔT, and ΔS emerged as significant within the model evalu- ations. The results suggested that high variation in salinity reduces mangrove habitat suitability for this species. Salinity variation along many of south Florida’s coastal bays is largely driven by water management-related freshwater canal discharges. Everglades restoration efforts seek to reduce ΔS along south Florida’s mainland shoreline; thus, if successful, restoration may also confer benefits to Scarus guacamaia through the subsequent expansion of suitable mangrove habitat. KEY WORDS: Conservation · Logistic regression · Habitat suitability · Coral reefs · Ontogenetic shifts Resale or republication not permitted without written consent of the publisher INTRODUCTION dangered Species Act (National Marine Fisheries Service 2006), there is concern that declines in rain- Rainbow parrotfish Scarus guacamaia is the largest bow parrotfish populations will link to reductions in herbivorous fish in the Atlantic Ocean and Carib- algal grazing and therefore further accelerate coral bean Sea and is the only scarid that possesses an ob- declines (National Marine Fisheries Service 2011). ligate, functional dependence on mangroves for ju - Mumby & Hastings (2008) suggested that parrot- venile habitat (Dorenbosch et al. 2006, Mumby 2006, fishes, and specifically Scarus guacamaia, might en - Nagelkerken 2007). Rainbow parrotfish are listed as hance coral recovery through grazing on macroalgae ‘vulnerable’ under International Union for the Con- (see also McClanahan et al. 2002). S. guacamaia feed servation of Nature (IUCN) criteria (Ferreira et al. mainly by scraping epiphytic macroalgae from coral 2008), due to a reduction in suitable habitat and the structures (Bellwood et al. 2004), and high levels of reduced size of individuals due to fishing pressure. parrotfish grazing have been shown to lead to a With the listing of 2 acroporid corals Acropora pal - 2-fold increase in the recruitment of corals, such as ma ta and A. cervicornis as threatened under the En - Porites spp. and Agaricia spp. in the Bahamas (Mum - *Corresponding author. Email: [email protected] © Inter-Research 2012 · www.int-res.com 88 Aquat Biol 15: 87–98, 2012 by & Hastings 2008). Although the linkages between from habitats (e.g. Sampson & Al-Jufaily 1999, Fran- rainbow parrotfish and healthy coral reefs are com- sen et al. 2006, Bi et al. 2007). Predictive habitat mod- plex, the indications that rainbow parrotfish may eling also allows the assessment of the geographic improve reef health give impetus for defining critical distribution of a species to visualize patterns across, factors in rainbow parrotfish life history. and affecting, their range and to elucidate factors Scarus guacamaia exhibit an ontogenetic habitat that may influence their life history that are not read- shift: juveniles reside along mangrove-lined shore- ily quantified through other methods. lines before moving to coral reefs as they mature South Florida water management regime changes (Dorenbosch et al. 2006). The biomass of rainbow have substantially altered the habitat for Scarus gua- parrotfish more than doubles when coral reefs are camaia in southern Biscayne Bay by either diverting located adjacent to rich mangrove resources, defined more water through canals (as opposed to groundwa- as coverage of fringing red mangrove Rhizophora ter) or by altering the volume and frequency of fresh- mangle of ≥35% within a region (Mumby 2006). This water releases, creating drastic fluctuations in salin- functional dependency appears reinforced by evi- ity and nutrient loads (Serafy et al. 2007). Therefore, dence that local population extinctions of S. guaca- it is critical to define the current juvenile habitat prior maia in the Belize barrier reef system corresponded to further anthropogenic changes to this ecosystem. with the removal of mangrove stands (Mumby et al. Furthermore, because S. guacamaia may provide an 2004). The South Florida ecosystem once supported important ecosystem service for threatened corals on substantial mangrove habitats, but with increasing the Florida reef tract, defining juvenile habitat for the human population density, habitat alteration, and species may be an important accessory step in the water regime changes, the mangrove coverage in protection of coral resources. the region has declined by 80% (Snedaker & Biber 1996). In addition, due to the long-term anthropo - genic disturbance of this ecosystem, optimal habitats MATERIALS AND METHODS defined within the Biscayne Bay region may no longer represent true ecological baselines. Although Study region substantial mangrove habitat remains — 1500 km2 within the Everglades (Olmsted & Loope 1984), in - The focal lagoon system comprised southern Bis- cluding an estimated measurement of 125 km2 within cayne Bay from Key Biscayne to Bar nes Sound the Biscayne Bay study area alone — only a few areas (Fig. 1). Northern Biscayne Bay’s water shed is heavily appear to support juvenile rainbow parrotfish (Serafy developed, and its once- mangrove- lined shore lines et al. 2003). Thus, it is likely that it is not solely the have mostly been re placed by seawalls and rip-rap presence of mangroves but rather a suite of other (limestone boulders). Estuarine-like characteristics abiotic or biotic factors that define habitat suitability occur only near points of freshwater inflow during the for S. guacamaia. These factors may include salinity, wet and early dry season (Wang et al. 2003). Southern temperature, dissolved oxygen, average depth, and Biscayne Bay generally averages <3 m in depth, with distance from offshore channel openings that present few deeper areas, and mainly small cuts and inlets to physiological and physical constraints on the life his- the ocean. This lagoon system leads to highly spatio - tory of this species. temporally variable salinity re gimes due to limited In the present study, we develop a logistic regres- tidal flushing and high fresh water input through sion model to predict the presence and absence of canals with gated control structures. The greatest rainbow parrotfish in mangrove habitats of southern salinity fluctuations occur near anthropogenic canal Biscayne Bay, South Florida, USA, with the goal of mouths along the western margin of southern Bis- better defining the characteristics that make a given cayne Bay, while the smallest fluctuations occur near mangrove habitat suitable for juvenile rainbow par- the inlets and passes (Wang et al. 2003), which are rotfish. Biscayne Bay’s watershed, from north to buffered by connections to the ocean. south, grades from urban to suburban to agricultural Southern Biscayne Bay is predominantly lined by and finally serves primarily as parkland. We use data red mangroves on both the mainland and the lee- collected from a long-term (>10 yr) visual survey of ward sides of the keys that define the bay’s eastern mangrove fishes from southern Biscayne Bay. To the border (Serafy et al. 2003), and mangroves are also extent that presence-absence defines habitat suit- the dominant shoreline habitat type of the region. ability, logistic regression is a valuable tool for deter- Red mangrove stands occur in narrow bands transi- mining the factors that harbor or exclude animals tioning to black mangrove Avicennia germinans and Machemer et al.: Mangrove habitat suitability for Scarus guacamaia 89 Fig. 1. Scarus guacamaia. Location and size of observed rainbow parrotfish individuals based on Mangrove Visual Survey (M, black circles, 1998−2007) and Reef Visual Census (R, red circles, 1979−2005) data. Legend in figure gives fish size (in mm), number of individuals seen (in parentheses), and survey (M or R) then white mangrove Laguncularia racemosa with the Mangrove Visual Survey (MVS) (see full descrip- in creasing distance from the shoreline. Scarus gua- tion in Serafy et al. 2003). Data from 1998 to 2007 camaia occurs in low abundance in the Biscayne Bay were used as the ‘training’ data to develop the model, region, in comparison to other more prevalent teleost and data from 2008 to 2009 were used to test the families, such as Haemulidae and Lutjanidae, but resulting model. The spatial extent of the MVS is this species is more abundant here than in other parts shown in Fig. 1. This