• Abstract and Figures
• Public Full-text

Competition between the comber (Serranus cabrilla) and painted comber (Serranus scriba) at STARESO, Corsica, France. Anja Sjostrom and Ryan Stephenson Abstract Interspecies competition is occurring between two Serranids; Serranus scriba and Serranus cabrilla at STARESO in the bay of Calvi, Corsica. Normally partitioned by depth, the two species overlap in resource and habitat use in this area. Our goal was to identify differences and similarities between the two species, determine the extent to which their habitats and resources overlap, and establish how they are partitioning them to avoid constant competition. 37 fish specimen were examined in the lab for stomach content and jaw morphology. Available habitat and selective usage by the two species was established by uniform point contact (UPC) and opportunistic surveys noting size, depth, substrate and cover habitat of 290 S. scriba and 39 S. cabrilla were taken on SCUBA. Our data show the two species consuming highly similar prey items, and having little difference in jaw morphology. The two species are partitioning substrate, cover and depth within the study site. We conclude that competition is occurring at STARESO, and that S. scriba and S. cabrilla are partitioning habitat to avoid resource overlap. Introduction Serranus scriba, the painted comber, and Serranus cabrilla, the comber, are two species of grouper that coexist in the nearshore rocky reefs off Corsica, France. Though similar in trophic level and morphology, both species tend to segregate by depth making co-occurrence uncommon. According to Dr. Pierre Lejeune (pers., Comm), the research director at STARESO, S. scriba have had a long and stable population in the area whereas S. cabrilla was present in the area until 1995, when the population became severely depleted. This deep population of S. cabrilla used to be found in 50-80m depths. In 2000, S. cabrilla began to reappear in the area. Serranus scriba are found in depths of 0-30m, whereas S. cabrilla occupy depths of 20-500m (Lythgoe & Lythgoe, 1975). At STARESO, S. cabrilla can be seen in shallower depths, and overlapping with the local population of S. scriba. Despite the species normal depth segregation, the two species share similar habitat and dietary preferences, indicating high potential for interspecies competition (Fasola et al., 1999). Serranus scriba and S. cabrilla are small fish of the family Serranidae and range across the Mediterranean and Eastern Atlantic and are true hermaphrodites. Serranus scriba lives over seagrass beds and rocky reefs and is territorial and solitary. Serranus cabrilla is the less abundant of the two and is also a solitary species occupying sand bottoms and rocky reefs (Guidetti, 2000). We know that the diets of the two species are similar (Lythgoe & Lythgoe, 1975) but we expect that there will be small scale changes in the diet due to competition. Interspecific competition between such sympatric species has been well documented in rocky reefs off the California coast (Holbrook & Schmitt 1988; Larson, 1980) but has not been addressed for these two species. We believe that both species may be competing for resources and space. In a study in California (Larson, 1980), one species of rockfish was found to have established a more pronounced dominance in the overlapping habitats than the subordinate rockfish. This may be present in the competition occurring between S. scriba and S. cabrilla. With limited background information on comparative ecology between S. scriba and S. cabrilla, the purpose of this study was to a) determine whether both species are partitioning resources, and b) whether both species are partitioning habitat. To answer these questions, we conducted diet analysis and morphological comparisons in the lab and used SCUBA to determine small scale spatial habitat use in the field. We expect that dietary composition will vary between the two Serranids species, and that they will characteristically differ in their utilization of the Posidonia and rocky reef assemblages for foraging and shelter at STARESO. Methods and Materials Species Description Apparent habitat and diet similarity between S. scriba and S. cabrilla present a valuable opportunity to examine resource and habitat partitioning of sympatric species. Previous work on Mediterranean rocky slope fish assemblages indicates the species partition their habitat by depth (Fasola et al 1997) and generally S. scriba utilizes Posidonia oceanica meadows more as habitat than and S. cabrilla. This makes STARESO an ideal study site, with meadows extending past the depths that stratify the two species, forcing S. cabrilla up into the shallows dominated by S. scriba. Both species are relatively common at STARESO, where they utilize nearshore rocky reefs as habitat. Their diets consist mainly of small crustaceans and other fish (Fasola et al., 1999). Both species have relatively small home ranges and are simultaneous hermaphrodites, breeding in the summer. Site description We gathered all observational data on SCUBA at STARESO field station, Calvi, Corsica in the northwest Mediterranean Sea (N 42.581795, E 8.725128). We collected data north and south of Stareso harbor in Posidonia oceanica meadows and rocky reef slopes between 3 and 14 meters (Figure 1). Lab Methods Resource partitioning is occurring between Serranus scriba and Serranus cabrilla. Gut Content Analysis To examine if diet composition differs between S. scriba and S. cabrilla, we collected individuals of both species for gut content analysis by spearfishing. We emptied the contents of the stomach onto a petri dish marked with a twenty-five point grid. We counted all contents in a standardized way. At each of the grid’s intersections, prey was identified as fish, crustacean, mollusk or cephalopod. By tallying all the points for each prey type, the percent composition that each prey type represented was determined. Jaw Measurements In order to test whether jaw morphology is dissimilar in S.scriba and S.cabrilla we took measurements to provide an illustration of potential prey differences between the two species. Standard length and width of each specimen was taken prior to gut removal and analysis. To test for differences in jaw morphology between the two serranid spp. vertical and horizontal gape measurements were taken. All measurements were recorded in millimeters. Field Methods Serranus scriba and Serranus cabrilla are partitioning habitats and depth in the mediterranean rocky reefs and posidonia fields. UPC and Focal data surveys To examine habitats of both species, we conducted uniform point contact (UPC) and opportunistic individual focal surveys to quantify comparative habitat features and resource use due to competition. We conducted surveys to the north and south of STARESO harbor. To characterize substrate habitat type, UPC counts were taken along 30m transects placed in areas of habitats known to be occupied by Serranus spp. We used the following substrate categories: sand, cobble (1-10cm), boulder (rocks larger than 10cm unattached to substrate), bedrock, and other. Cover layers were grouped in the following manner: turf=less than 3cm, bushy=greater than 3cm, foliose, erect coralline, encrusting coralline, gunk=film layer less than 1cm, sessile invert, other or none. To gather available cover and substrate data for the northern portion of the study area we used nine point quadrats (0.5m x 0.5m)off a permanent transect line. At 5 meter intervals we ran 30 meter transects off of the permanent line and placed the quadrat down for sampling at 5 meter intervals on either side of the transect tape. We counted substrate and cover under each of the quadrat’s nine points. To quantify how both species were associating with the different habitats we conducted opportunistic focal data surveys each S.scriba or S. cabrilla encountered. Each fish was visually sized to the nearest centimeter and the depth it was sighted at recorded in meters. We recorded the focal data of each fish by their associated substrate and cover layer as defined by the UPC categories. Surveys were conducted from October 22th until October 30th 2012. Statistical Analysis Gut content was characterized by the number of diet items in each category and standardized to total length of the fish. Data were analyzed for similarity (Bray-Curtis distances) in PRIMER. Horizontal and vertical gapes were divided by the standard length of each specimen to standardize gape size for each fish. We then used these standardized gapes to analyse data for similarity (Bray Curtis distances) in PRIMER to determine similarity in jaw morphology of S. scriba and S. cabrilla. Chi square tests were run to compare the availability of each substrate and cover type to the observed use by S.scriba and S.cabrilla. ANOVA tests were run to analyze the available depth range and quantify any depth stratification between the two species. Results We must accept the null hypothesis that the diets of the two species do not differ (Figure 3, p<0.564). Many of the replicates from the analyses ate the exact same food type as another replicate, which gives the appearance of a lesser number of fish. We must accept the null hypothesis that there is no difference in the maximum vertical and horizontal gape between the two species Figure 3: The similarity in jaw morphology (Figure 3, p<0.964). S. (maximum horizontal and maximum vertical gapes cabrilla exhibits a strong divided by standard length) between S. scriba and positive association with S.cabrilla. Green triangles depict jaws from S. scriba; bedrock, utilizing it upside-down blue triangles are from S. cabrilla. significantly more than the There is no difference between the jaw available quantity (Figure 4, morphologies of the two species. p<0.001). They showed a strong negative association with the relatively abundant Posidonia, with only two out thirty-nine individuals utilizing it as habitat (Figure 5, p<0.001). Negative associations with boulders reflect the high abundance of boulders in the survey area and the low abundance of S.

Load more

  10

Copy Link