Inter- and intra-habitat movement patterns and population dynamics of small reef fishes of commercial and recreational significance

This research examined smaller reef fishes of commercial and recreational fishing significance (i.e. snappers, emperors and groupers) on the Great Barrier Reef (GBR), Australia. Four questions of importance to reef fish ecology and reef fisheries science were addressed. These were i) identification of patterns of distribution and abundance; ii) quantification of local patterns of inter- and intra-habitat movements of post-settlement reef fishes in six habitat types in a lagoon; and for Lutjanus fulviflamma, Lethrinus harak and L. lentjan, iii) the estimation of age, growth and mortality rates and iv) estimation of size and age at first sexual maturity. Antillean Z-traps were used to determine patterns of distribution and abundance of reef fishes in six types of habitat in a lagoon at Lizard Island, GBR. The habitats were 1) deep sand away from reefs (DSAR), ii) deep sand near reefs (DSNR), iii) rubble areas (RUBB), iv) slopes of reefs (SLOP), v) shallow sand near reefs (SSNR) and vi) tops of reefs (TOPS). Variations in catch composition and abundance between day and night soaks in these 6 habitats were measured on 7 sampling occasions over a period of 30 months. Data were analyzed using a 3-way fixed ANOVA model (factors were 7 sampling occasions, 6 habitat types and 2 soak times). Habitat types and soak time were the most important factors in explaining variation in catch composition and abundance for most species of reef fish. In general, abundance and species richness were significantly higher at the TOPS and SLOP habitats and lower in the SSNR and DSAR habitats. The DSNR and RUBB habitats had intermediate numbers of species and individuals. DSNR and RUBB habitats were more similar to TOPS and SLOP habitats in terms of species composition than to SSNR and DSAR but were more similar to SSNR and DSAR in terms of abundances. Overall, more individuals and species were caught during night than day in all habitats except in TOPS. The abundances of many lutjanids and lethrinids were higher at night than day in sandy habitats (DSNR, DSAR and SSNR). The abundances of apogonids and holocentrids were higher at night than day in reefal habitats (TOPS and SLOP). In contrast, the abundances of pomacentrids and labrids were significantly higher during the day than at night in reefal habitats. A TWo-way INdicator SPecies ANalysis (TWINSPAN) on the catch information (120 species by 84 samples of 6 replicate traps; i.e. 7 sampling times by 6 habitat types by 2 soak times) revealed two distinct fish assemblages associated with a) reefal and rubble, and b) sandy habitats. These assemblages changed over a diel period with each of the two 'habitat-based' assemblages showing a distinct day and night fish composition. The nocturnal fish assemblage in sandy habitats further differentiated into two groups, one near and the other away from reefs. The results indicated the importance of habitats in the local distribution of reef fishes. The present study confirms the significance of habitats as a source of shelter and food resources for reef fishes. The shifts in fish composition between day and night suggested differential movement patterns of fishes from hiding places to feeding areas. Many of the lutjanids, apogonids and holocentrids hid whilst pomacentrids and labrids foraged in reefal habitats by day. By night, pomacentrids and labrids sought shelter whilst apogonids and holocentrids foraged within reefal habitats. Some lutjanids moved to sandy habitats near reefs, while others moved further away from reefs at night to forage. The sandy habitat appeared depauperate during the day but many species, particularly Lutjanus fulviflamma, Lethrinus lentjan and L. nebulosus, frequented this habitat at night. A mark-recapture technique was used to determine levels and patterns of movement within and between habitat types for lutjanids, lethrinids and serranids. Distances moved were categorized in intervals of 30 m, based on the minimum distance between deployed traps. Results showed that 74% of movements (n=286) were within and 26% were between habitats. These fishes exhibited strong habitat fidelity except in the shallow TOPS habitat, and a high propensity for short distance movements of 30-60 m. Movements within habitats greater than 100 m comprised 20% of the total and only 5% of movements were greater than 500 m. Movements between habitats greater than 100 m comprised 42% of the total and only 5% of movements were greater than 500 m. Large distance movements (100-1500 m) across vast expanses of deep sand were recorded for some reef species (e.g. Lethrinus nebulosus, Lutjanus carponotatus, and L. fulviflamma), but were rare. Lethrinus nebulosus appeared to move larger distances than the other species. The number of movements during the night was significantly higher in SLOP and DSNR habitats than during the day but the distances moved did not differ between night and day. These findings are relevant to the design and location of marine reserves as management tools in coral reef fisheries. Knowledge of movement patterns of reef fish is essential in measuring flux rates of reef fishes across reserve boundaries to adjacent fished areas. Mark-recapture and aquarium experiments combined with tetracycline (OTC) injection were used to validate the periodicity of opaque bands in sectioned otoliths of Luanus fulvifiamma, Lethrinus harak, and L. lentjan. Three experiments were conducted on a total of 57 fishes covering a wide range of ages and sizes. Of these, 10 Lutjanus fulviflamma, 8 Lethrinus harak and 15 L. lentjan survived long enough to provide useful information. Results showed an opaque band outside the tetracycline mark and well below the otolith margin in sectioned otoliths of specimens surviving more than a year after OTC treatment (3 of 10 Lutjanus fulviflamma and 3 of 15 Lethrinus lentjan) or for those which survived from July to March (2 L lentjan). In the case of Lethrinus harak no specimen survived more than a year post OTC treatment. The periodicity of formation of opaque bands in otoliths of L. harak was assessed indirectly by examination of the distance between the OTC mark and the otolith margin. This distance represented a known period of otolith growth. It corresponded to a fraction of the distance between any two consecutive opaque bands in older fishes (after the third band). This fraction was proportional to the survival period of fish after OTC injection and suggested that the distance between two consecutive opaque bands was roughly equivalent to a year's growth (in 4 of 8 L. harak). This suggests that the opaque bands in L. harak were formed once each year. Thus, the opaque bands in sectioned otoliths of Luanus fulviilamma, Lethrinus lentjan and L. harak were confirmed to be annuli and were determined to be laid down during the months of August to December at Lizard Island, GBR. Age determination of fishes based on the validated counts of opaque bands in sectioned otoliths showed a high percentage of agreement and a low index of average percentage error (IAPE) among readers, demonstrating the technique to be highly reliable, precise and accurate. Estimates of von Bertalanffy growth parameters (L(∞) (mm FL), K and t₀, respectively, ± SE) obtained using the above method were 246.3 (±3.5), 0.261 (±0.037) and -4.377 (±0.640) for Lutjanus fulviflamma (n=176), 285.0 (±5.0), 0.313 (±0.050) and -3.159 (±0.567) for Lethrinus harak (n=132) and 307.2 (±6.8), 0.345 (±0.047) and -2.202 (±0.290) for L. lentjan (n=117). Maximum age observed for Lutjanus fulviflamma was 17 years, 15 years for Lethrinus harak and 14 years for L lentjan. Estimates of natural mortality rate (M ±sE) from age-based catch curves were 0.231 (±0.035) for Lutjanus fulviflamma, 0.381 (±0.097) for Lethrinus harak and 0.305 (±0.078) for L lentjan. Thus, these three species were long lived, slow growing (but with an initial phase of rapid growth) and had low rates of natural mortality. This information has important implications to the management of the fishery of these species in the future. Their life history characteristics imply that they may be vulnerable to intense exploitation. Histological examination was performed on gonads of Lethrinus harak (n=131), L. lentjan (n=96) and Lutjanus fulviflamma (n=94; females only) to assess stages of oocyte development. Age and size at first sexual maturity for the 3 species were determined as where 50% of samples in an age or size class attained maturity. Results showed that Lethrinus harak reached sexual maturity at 2 years of age and at a size of 220-229 mm FL, L. lentjan at 3 years and 250-259 mm FL and Lutjanus fulviflamma at 2-3 years and 200-209 mm FL. The presence of an ovarian lumen, brown bodies, the lobed arrangement of spermatogonia, the thicker gonad wall of younger males, and the female biased sex-ratios at younger ages and at smaller sizes were evidence consistent with protogynous hermaphroditism for Lethrinus harak and L. lentjan. Lutjanus fulviflamma was gonochoristic. This information forms an important basis for setting legal minimum size limits for these species on the GBR. The information gained in this study is highly relevant to coral reef fisheries management. The data on movement patterns of reef fishes are particularly useful in testing the utility of marine reserves as a management option. The information on the life history characteristics of Lutfanus fulviflamma, Lethrinus harak and L. lentjan provide the first data for these species on the GBR. This research stresses the need for age-based methods of estimating important life history characteristics of other species in the future