Burns & Kopecky, 2014 1 an Investigation of the Presence Of
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Burns & Kopecky, 2014 1 An investigation of the presence of Mediterranean Moray Eels (Muraena helena) within rocky reefs and seagrass meadows (Posidonia oceanica) in Corsica, France S. Burns and K. Kopecky Department of Ecology and Evolutionary Biology, University of California, Santa Cruz 2014 ABSTRACT The Mediterranean Moray Eel, Muraena helena, is widely distributed throughout the Mediterranean and plays a significant ecological role as a part-time resident top predator within the rocky reef ecosystem. While current literature suggests that M. helena stay close to their home site within the rocky substrate, conflicting observations have shown that M. helena may be abundant within both rocky and sea grass (Posidonia oceanica) habitat. This study investigated the extent to which M. helena is found in both rocky and seagrass habitat near Corsica, France. We also estimated the local population size and explored ontogenetic relationships among certain morphometric measurements. We found M. helena captures are significantly more likely in P. oceanica meadows than in the rocky habitat. We propose that M. helena travel further from their home site than previously assumed and that P. oceanica serves as a foraging site in addition to the rocky habitat. Furthermore, we found that the vertical gape of these eels grows disproportionately faster than other head dimensions, which may contribute to their success as apex predators within the Mediterranean. INTRODUCTION Moray eels (Muraenidae) comprise a family of 200 predatory fishes that are widely distributed in many temperate and tropical waters throughout the world. The cryptic and aggressive nature of this taxon is somewhat responsible for the limited knowledge on their general biology, ecology, and behavior (Reece et al. 2010). The Mediterranean Moray Eel, Muraena helena, is common found throughout the Eastern Atlantic and the Mediterranean Sea. Even though M. helena plays a significant role in the Mediterranean rocky reef ecosystems as a top predator, only a few studies exist that have further examined the behavior, abundance, and ecological role of these eels (Hixon et al. 2012, Matić-Skoko et al. 2010). Mature M. helena are skilled nocturnal predators that predominantly feed on benthic fish, crustaceans, and cephalopods (Gothel 1992). They primarily inhabit crevices within rocky habitat during the day and the literature suggests that M. helena do not forage far from their home dens at night (Böhlke et al. 1989). Initial observations in October 2014 at the Station Research Sous-Marine et Oceanographique (STARESO), located in Calvi, Corsica, suggest that M. helena may be found in both rocky and seagrass (Posidonia oceanica) habitats. We investigated the extent to which M. helena is found in both rocky and P. oceanica habitat in the subtidal waters of Corsica, France. We hypothesized that overall abundances of M. helena would be different between rocky and P. oceanica habitat. We also tested the hypothesis that the population of M. helena in 2014 at STARESO will have changed from the population estimated by Moffit & Gervais in 2012. Additionally, we asked: 1. What is the spatial distribution of M. helena, with respect to north/south directionality and individual trap location within each habitat at STARESO? Burns & Kopecky, 2014 2 2. What is the age distribution of M. helena with respect to habitat, north/south directionality and individual trap location within each habitat at STARESO? 3. Is there an allometric relationship between certain morphometric measurements with respect to age in M. helena? METHODS Study System We conducted this study in October 2014 at Station Research Sous-Marine et Oceanographique (STARESO), Point Revellata, Calvi, France. The study location provided a suitable research site due to the abundance of M. helena in shallow waters and the dichotomy of substrate (rock and seagrass) found throughout the nearby marine ecosystem. Sampling Design We set 10 baited traps slightly north of the STARESO harbor. 5 traps were set in 7-10 meters of water on rocky substrate while 5 adjacent traps were set in P. oceanica at 10-13 meters of depth. Traps set in seagrass were placed 20 m offshore of traps set on rocks. Adjacent traps within each habitat were set 30 m apart (Fig. 1). Substrate, depth, and trap location were measured and marked on SCUBA. Each trap was baited every night with local fish (Labridae, Sparidae, Serranidae) and/or Octopus vulgaris. We trapped 16 hours each night for 15 nights, from 17:30-09:30, totaling 240 trap hours. We used a previously determined individual trap range to estimate the total area that our Fig 1. STARESO harbor with trap layout. traps were drawing from (Moffit & Gervais, 2012). Rock traps are depicted in green, while seagrass traps are depicted in white. Morphometric Data Collection Captured M. helena were anesthetized with 1ml of clove oil to 5 liters of sea water. Total length, standard length, head length, head width, head height, body width, body height, and vertical gape were measured to the nearest millimeter. We palpitated each individual to remove and record gut content. We then recorded the eel’s mass to the nearest gram. We used one- sample linear regressions to analyze relationships between body metrics. Mark-Recapture Analysis We threaded a unique combination of 3 colored beads on the dorsal fin to mark and identify each captured individual. After reviving the anesthetized individual in seawater, it was released near the site of capture. Based on our recapture data, we calculated the local population with the mark-recapture equation: N = - N1/(P – 1), where N is the calculated population size, N1 is the number of individuals caught at the time of the first recapture, and P is the probability of catching a new eel, which is the number of unique individuals caught divided by the total number caught (including recaptures). Burns & Kopecky, 2014 3 Age Analysis The age of each captured moray was estimated with a von-Bertalanffy growth function, −k(t-t0) TL = L∞(1 − e ), where TL is total length at age t, L∞ is the asymptotic length at which growth is zero, k is the body growth coefficient, and t0 is the theoretical age at zero length. These −1 parameters (L∞ = 162.7 cm, k = 0.089 yr , t0 = −0.660 yr) were determined by Matić-Skoko et al. (2011) by correlating the total lengths of M. helena individuals captured in the Adriatic Sea to ages obtained from otolith measurements. We rearranged the von-Bertalanffy equation so as to input our measured total lengths and output the corresponding age for each individual we captured: t = t0 – [ln(1-TL/L∞)]/k. Comparative Abundances To compare abundances of M. helena between habitats, we calculated and compared daily capture rates. To assess the relationship between captures, directionality (north-south), and individual trap location, we ran one-sample linear regressions. Finally, we used binomial probability to compare the likelihoods of capturing an eel in either habitat. RESULTS Comparative Abundances We trapped a total of 37 morays over 12 successful trapping days. 13 individuals were caught in the traps placed on rocky habitat, while 24 individuals were caught in the traps set within the seagrass. The daily capture rates between the two habitats did not differ significantly. However, we found it significantly more likely to catch an M. helena individual within the P. oceanica than in the rocky habitat (p = 0.0494). Population Estimate and Comparison Of the 37 individuals we marked, we recaptured only 5. We failed to note the date of our first recapture and had to estimate our N1 value, and thus the population size, via a resampling program. The most probable population size in the area we trapped at STARESO was 66.33 individuals, with a range of 37 – 88 individuals. We estimated our trapping area as 8027 m2. Our catch per unit effort was 0.15 eels/hour. We found a 2.4-fold increase in trapping success when comparing our catch per unit effort to 0.0625 eels/hour, as reported by Moffitt & Gervais (2012). Associations to Directionality Because our trap layout inherently represented a south-north gradient, we examined the relationship, if any, between captures and directionality. The rock traps showed a non- significant trend of higher trapping success in the north, while the seagrass traps showed a significant trend of higher trapping success in the south (p = 0.0287, Fig 2). We found no significant correlations of age to either directionality or individual trap location. Fig 2. Sum of captures vs. trap location (block). 1 represents the southernmost trap, while 5 represents the northernmost trap for each habitat type. The data for the rock traps are plotted in red (p = 0.117, R2 = 0.61, DF = 4). The data for the P. oceanica traps are plotted in blue (p = 0.0287, R2 = 0.84, DF = 4). Burns & Kopecky, 2014 4 Ontogenetic Morphometric Analysis After calculating age with the rearranged von Bertalanffy growth function, we plotted our data on a model curve of this function, overlaid with the raw values for age and total length from the Matić-Skoko et al. (2011) study (Fig. 3a). a) b) Fig 3a. Total length as a function of age. The green model curve represents the von Bertalanffy growth function. The Adriatic data of the Matić-Skoko et al. (2011) study are overlaid as empty blue circles. Our data, with only raw values for length, are represented by black triangles. Fig 3b. The ratio of head-length/vertical gape as a function of age (p = 0.001, R2 = 0.27, DF = 36). The regressions of head measurements/vertical gape against age showed that vertical gape grows disproportionately faster than any other head dimension. Most significantly, vertical gape grows at a faster rate than head length, indicated by the negative slope (p = 0.001, Fig.