Transactions of the Royal Society of South Australia (2013), 137(1): 80–89
CONTRASTING DIET OF TWO TEMPERATE REEF FISH SPECIES (NOTOLABRUS TETRICUS AND MEUSCHENIA HIPPOCREPIS) AS DETERMINED FROM COMMERCIAL ROCK LOBSTER BYCATCH SAMPLES Giverny G. rodGers1*, AdriAn J. LinnAne2 & ChArLie huveneers1,2 1. Biological sciences, Flinders university Po Box 2100, Adelaide 5001, south Australia. 2. south Australian research and development institute Po Box 120 henley Beach 5022, Adelaide, south Australia. * Corresponding author: Biological sciences, Flinders university Po Box 2100, Adelaide 5001, south Australia. email:
[email protected], Telephone: +61 8 8201 2280, Fax: +61 8 8201 3015.
Abstract
The commercial rock Lobster Fishery is south Australia’s most valuable wild fishery. its bycatch is primarily composed of finfish that includes wrasses, leatherjackets, and bearded rock cods. This study used bycatch samples to broaden the biological understanding of two such species, namely the bluethroat wrasse Notolabrus tetricus and the horseshoe leatherjacket Meuschenia hippocrepis. specifically, we examined the diets of each species through stomach content analyses, which were then compared to gape morphology. PerMAnovA analyses indicated a highly significant difference in diet between bycatch species. For M. hippocrepis, sponges, and red and brown algae were important dietary components, while molluscan prey species were more prevalent in N. tetricus. Morphometric analyses indicated that gape morphology was adapted to dietary items consumed. Mouth parts of M. hippocrepis were beak-like and highly suited for algal grazing. in contrast, mouth parts of N. tetricus consisted of a masticating plate combined with sharp, conical teeth that allowed for consumption of hard-shelled molluscan prey species. Keywords: bluethroat wrasse; horseshoe leatherjacket; stomach content; bycatch.
Introduction
The commercial south Australian rock Lobster Fishery (sArLF) is the state’s most valuable wild fishery (Knight & Tsolos 2010). in the 2010/11 season the number of recorded potlifts was ~1.6 million, with 90% of catches exported live overseas, primarily to China (Linnane et al. 2012). For the purpose of management, the fishery is divided into two fishing zones (the northern and southern zones), which are further sub-divided into Marine Fishing Areas (MFAs) (Fig. 1).
Previous studies have shown that bycatch in both the northern and southern zones of the sArLF is dominated by finfish catches, with wrasses (primarily blue throat wrasse, Notolabrus tetricus), leatherjacket species (primarily horseshoe leatherjacket, Meuschenia hippocrepis), and bearded rock cod (Pseudophycis barbata) being the most abundant (Brock et al. 2007). The estimated average finfish bycatch for the 2002/03 and 2003/04 seasons combined was 0.406 specimens potlift-1, with higher overall bycatch rates in the northern zone of the fishery compared to the southern zone (Brock et al. 2007).
Notolabrus tetricus and M. hippocrepis are relatively common in south Australian rocky reef habitats (Gomon et al. 2008). Notolabrus tetricus is a hermaphroditic species and has a limited home range (Barrett 1995). Females have largely overlapping home ranges, whereas males are territorial, particularly during the breeding season. Preferred prey items of N. tetricus sampled within south Australia have been found to include molluscs and crustaceans (shepherd & Clarkson 2001). while the species is targeted commercially and recreationally in south-eastern Australia, catches in south Australia are small with the commercial catch for 2010/11 being less than 25 tonnes for Notolabrus spp. (shepherd et al. 2009; Fowler et al. 2011). very little biological information is available for M. hippocrepis. This species occurs on temperate reef off southern 80
ConTrAsTinG dieT oF Two TeMPerATe reeF Fish sPeCies
Figure 1. south Australian northern and southern zone rock lobster fisheries with Marine Fishing Areas.
Australia to a depth of ~18 m and has a known maximum length of 60 cm (edgar 2008; Gomon et al. 2008). There is also a small commercial fishery for leatherjackets in south Australia, which is not reported to species level, with 11.6 tonnes landed during the 2010/11 season (Fowler et al. 2011). The aims of the study were twofold. Firstly, we aimed to expand on the previous work by Brock et al. (2007) by examining bycatch species composition and biodiversity within the northern Zone rock lobster fishery. secondly, having identified important bycatch groups, we investigated differences in diet between two key species which we hypothesised would correlate with apparent differences in gape morphology.
Sample collection and storage
Methods
The study was undertaken within the northern Zone rock Lobster Fishery (nZrLF), which extends from the western Australian border to the river Murray mouth in the Coorong region of south Australia (Fig. 1). sampling was carried out over two fishing seasons (2009/10 and 2010/11), within three of the ten most commonly fished MFAs: Area 40 off yorke Peninsula (yP) and Areas 39 and 49 off Kangaroo island (Ki) (Linnane et al. 2012; Fig. 1). All samples were collected during normal commercial fishing operations using standard baited rock lobster pots that were set at dawn and hauled ~24 hours later. All finfish bycatch was frozen at -30°C within 12 hours of capture.
on processing, bycatch samples were thawed and identified to species level. Fish total length and weight were measured and recorded to the nearest mm and 0.1 g, respectively. stomachs were removed and placed into a labelled and punctured ziplock bag before being transferred into a 20-litre bucket of 99% formalin solution made to a 2:3 formalin:seawater ratio for fixation. it was necessary to fix stomach contents in formalin to preserve soft-bodied organisms and to prevent shrinkage of samples which would affect later weight measurements. After a period of 48 hours, samples were washed in fresh water for at least six hours before being transferred to a bucket containing 99% ethanol as preservative. 81
Giverny G. rodGers, AdriAn J. LinnAne & ChArLie huveneers
Gut content analysis
samples were removed from the ethanol preservative and washed with fresh water in a 60 µm sieve to remove excess ethanol; before being transferred to Petri dishes for further examination. stomach contents were examined under a dissecting microscope and each recognisable prey item was identified to the lowest possible taxonomic level. each specimen group was placed in a 5-ml vial and the total weight (nearest 0.01 grams) of each group was recorded using an electronic balance. Data analysis
The species diversity (breadth) of bycatch was calculated using the combined index (Ci; Cortès et al. 1996). The combined index is derived from the average of the Levin’s index (B’) and the shannon-weiner index (H’) standardised on a scale of 0–1 (Krebs 1999). Length-weight relationships were estimated using power regression curves with significance of the regression analysis determined using analysis of variance (AnovA). Further analyses as described below were carried out for only the two most common bycatch species.
Cumulative prey curves were used to assess if the stomachs obtained were a good representation of the natural diet of bycatch species. The curve was constructed by randomising the stomach order ten times and calculating the average number of each of the prey items (Ferry et al. 1997). The cumulative number of prey was plotted against the number of stomachs examined. An accurate representation of bycatch diet was considered to have been obtained once the prey curve reached an asymptote. Cumulative curves were determined to be asymptotic if at least ten previous values of the total number of prey were in the range of the asymptotic number of prey ±0.5 (huveneers et al. 2007).
The percentage mass (%M) and percentage frequency (%F) were calculated for each prey type to consider the relative importance of each prey. using %M, diet diversity (breadth) was determined using the combined index (Ci; Cortès et al. 1996) for each species and location. in addition, diet overlap between species and locations was also determined using horn’s index (r0) (horn, 1966). Low overlap was considered to have a value of 0–0.29; medium of 0.3–0.59 and high of >0.6 (Langton 1982). A PerMAnovA was conducted with species as the fixed factor to determine if the diet of these two finfish bycatch was significantly different. A square-root transformation was applied to down weight highly abundant prey items, and Bray-Curtis similarities were used to remove any effects of joint absences. A total of 9,999 permutations of the residuals under a reduced model were used to calculate probability values. non-metric Multidimensional scaling ordination (nMds) was used to visually represent any patterns in the data. A siMPer analysis was used to examine the prey items contributing to the similarities and dissimilarities in the diet for each species. The above multivariate analyses were carried out using PriMer v6 and PerMAnovA+ for PriMer software package (Clarke & Gorley, 2006; Anderson et al., 2008). For all the statistical analyses, results were accepted as significant at the level of P < 0.05.
Bycatch species
Results
in total, 870 bycatch individuals were collected, with M. hippocrepis and N. tetricus being the most dominant species in terms of percentage of total bycatch (Table 1). Meuschenia hippocrepis was responsible for 66.67% of total bycatch, while N. tetricus made up 19.43% of bycatch. other finfish species present in the bycatch included other leatherjacket species (such as M. freycineti, M. scaber, and M. galii), brownspotted wrasse (N. parilus), and bearded rock cod (Pseudophycis barbata). Bycatch composition had a diversity of Ci = 0.19. As M. hippocrepis and N. tetricus made up the majority of bycatch composition, subsequent results are for these species only.
Bycatch diet
The stomach contents of 135 fish, 102 M. hippocrepis and 33 N. tetricus, were analysed with a total of 22 different prey groups identified. randomized cumulative prey curves for both bycatch species indicated that the prey items sampled were not completely representative of the diet of these bycatch species. despite this, 82
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Table 1. Total bycatch composition over both experimental seasons. Bycatch species
Meuschenia hippocrepis Notolabrus tetricus Meuschenia freycineti Meuschenia scaber Notolabrus parilus Meuschenia galii Pseudophycis barbata Chrysophrys auratus Tilodon sexfasciatus Centroberyx gerrardi Threpterius maculosus Caesioperca rasor Meuschenia australis
Total % of Bycatch
prey curves generally came close to reaching asymptotic stabilisation (Fig. 2). This was particularly true for the M. hippocrepis curve.
The most abundant prey item for M. hippocrepis in terms of %M was sponge (74.2 %) and in terms of %F was red algae (14.0 %). For N. tetricus, the prey item with the greatest %M and %F was molluscan shell (51.9 and 14.5 %, respectively) (Table 2).
66.67 19.43 5.06 4.14 1.38 1.26 1.03 0.34 0.23 0.11 0.11 0.11 0.11
Prey species diversity was lower for M. hippocrepis than for N. tetricus (Ci = 0.10 and 0.17 respectively). horn’s index gave a very low diet overlap between the two species (r0 = 0.10).
PerMAnovA results indicated a highly significant difference between species diet (F=25.03, P
