Tropical Demersal Research Articles

Community-scale relationships between body shape and trophic ecology in tropical demersal marine fish of northeast Brazil.

Functional morphology investigates the relationships between morphological characters and external factors, such as environmental, physical and ecological features. Here, we evaluate the functional relationships between body shape and trophic ecology of a tropical demersal marine fish community using geometric morphometrics techniques and modelling, hypothesizing that shape variables could partially explain fish trophic level. Fish were collected over the continental shelf of northeast Brazil (4-9°S). Analysed fish were distributed into 14 orders, 34 families and 72 species. Each individual was photographed in lateral view, and 18 landmarks were distributed along the body. A principal component analysis (PCA) applied on morphometric indices revealed that fish body elongation and fin base shape were the main axes of variation explaining the morphology. Low trophic levels (herbivore and omnivore) are characterized by deep bodies and longer dorsal and anal fin bases, while predators present elongated bodies and narrow fin bases. Fin position (dorsal and anal fins) on the fish body is another important factor contributing to (i) body stability at high velocity (top predators) or (ii) manoeuvrability (low trophic levels). Using multiple linear regression, we verified that 46% of trophic level variability could be explained by morphometric variables, with trophic level increasing with body elongation and size. Interestingly, intermediate trophic categories (e.g., low predators) presented morphological divergence for a given trophic level. Our results, which can likely be expanded to other tropical and nontropical systems, show that morphometric approaches can provide important insights into fish functional characteristics, especially in trophic ecology.

Trophic Structure and Diet of Predatory Teleost Fishes in a Tropical Demersal Shelf Ecosystem

Predatory fishes are a major component of many tropical fisheries, although little is known about their diet and trophic structure, which can hinder effective management. We used stable isotopes δ15N and δ13C in conjunction with dietary prey items of five fishes (Lutjanus sebae, Lethrinus punctulatus, Epinephelus areolatus, Epinephelus multinotatus, and Plectropomus maculatus) to describe the diet and trophic structure across this assemblage. A total of 153 isotope and 87 stomach content samples were collected at two locations that were ≈30 km apart, over two sampling trips, separated by three months. There was clear separation of species’ mean δ15N and δ13C values in isotopic space; the highest mean δ15N was exhibited by E. multinotatus (13.50 ± 0.11 SE) and the lowest was L. punctulatus (11.05 ± 0.13). These two species had the lowest overlap of isotopic niche space, whereas the highest overlap occurred between L. sebae and P. maculatus. δ15N increased with fish body size for all species. However, body size was not significantly related to δ13C values for any species. There was a notable shift in both δ15N and δ13C between sampling trips, with δ13C being more depleted in the second trip. There was also a difference in δ13C between locations for all species, suggesting localised foraging. A multiple tissue comparison for L. sebae indicated positive relationships for both δ15N and δ13C between dorsal fin and muscle tissue. Mean δ15N values were the same for both fin (12.1 ± 0.10 SE) and muscle tissue (12.1 ± 0.09 SE), although δ13C was more enriched in fin (-15.6 ± 0.14 SE) compared to muscle tissue (-17.3 ± 0.11 SE). The most common dietary items across species were teleosts and crustaceans, which was consistent with isotope data indicating a reliance on a demersal food web (δ13C values ranging from -18 to -15‰). The results from our study indicate a dynamic spatio-temporal trophic structure and diet for commercially important demersal species and highlight the benefits of a multi-facetted sampling approach.

Review of the life history characteristics, ecology and fisheries for deep-water tropical demersal fish in the Indo-Pacific region

Deep-water tropical fishes support locally significant commercial fisheries, high value recreational fisheries, and culturally and economically important artisanal and subsistence fisheries throughout the Indo-Pacific region. The main species captured by these fisheries are deep-water snappers (Lutjanidae), groupers (Epinephelidae), and emperors (Lethrinidae). Quantitative assessments of deep-water tropical fisheries have been limited by a lack of adequate biological and fisheries data. We review the biology and ecology of deep-water tropical fishes, discuss the implications for assessment and management of tropical deep-water fisheries, and provide perspectives on future research priorities. We found that biological and fisheries information is lacking for the majority of deep-water tropical fishes. Furthermore, many studies were constrained by low samples sizes and the use of methods that have not been validated or verified. Most species for which reliable information was available were reported to have extended longevities (>20 years), low rates of natural mortality (M < 0.15), and slow to modest growth rates (K = 0.12–0.25). These life history traits indicate a low production potential for many deep-water tropical fishes, and suggest that sustainable exploitation rates and potential yields may be low. There is a need for more representative and adequate studies of deep-water tropical fishes and for improved fisheries data collection and the use of consistent methods in addition to information sharing to facilitate the development of robust data-poor assessment techniques for these species.

Target strength estimates of red emperor (Lutjanus sebae) with Bayesian parameter calibration

Red emperor (Lutjanus sebae) is a long-lived tropical demersal snapper which is widely distributed in the Western Pacific and Indian Ocean. Despite the commercial and recreational importance of the species for the Northern Demersal Scalefish Fishery off the Northwest coast of Western Australia, we still lack a thorough understanding of its distribution and abundance in the area. To better understand the acoustic scattering properties of red emperor its acoustic backscattering characteristics were modelled based on swimbladder and body morphology, determined using computed tomography scans. A Kirchhoff-ray mode approximation was coupled with empirical (ex situ) measurements of target strength (TS) obtained from a 38 and 120 kHz split-beam echosounder on board a fishing vessel. Bayesian methods were used for model parameter calibration, which provided uncertainty estimates for some of the TS-model parameters. The derived TS-length relationships were 19.7log 10(L) – 75.5 (C.I. 5.9 dB) at 120 kHz and 14.6 log10(L) – 64.9 (C.I. 5.8 dB) at 38 kHz. The study demonstrated that small commercial fishing vessels can be used to conduct ex situ experiments and target strength modelling can be effectively based on computer tomography scans. This relatively low cost approach could be applied to other species. Red emperor (Lutjanus sebae) is a long-lived tropical demersal snapper which is widely distributed in the Western Pacific and Indian Ocean. Despite the commercial and recreational importance of the species for the Northern Demersal Scalefish Fishery off the Northwest coast of Western Australia, we still lack a thorough understanding of its distribution and abundance in the area. To better understand the acoustic scattering properties of red emperor its acoustic backscattering characteristics were modelled based on swimbladder and body morphology, determined using computed tomography scans. A Kirchhoff-ray mode approximation was coupled with empirical (ex situ) measurements of target strength (TS) obtained from a 38 and 120 kHz split-beam echosounder on board a fishing vessel. Bayesian methods were used for model parameter calibration, which provided uncertainty estimates for some of the TS-model parameters. The derived TS-length relationships were 19.7log 10 (L)-75.5 (C.I. 5.9 dB) at 120 kHz and 14.6 log 10 (L)-64.9 (C.I. 5.8 dB) at 38 kHz. The study demonstrated that small commercial fishing vessels can be used to conduct ex situ experiments and target strength modelling can be effectively based on computer tomography scans. This relatively low cost approach could be applied to other species.

Correction: Multi Year Observations Reveal Variability in Residence of a Tropical Demersal Fish, Lethrinus nebulosus: Implications for Spatial Management

Correction: Multi Year Observations Reveal Variability in Residence of a Tropical Demersal Fish, Lethrinus nebulosus: Implications for Spatial Management

Effects of mesh size and towing speed on the multispecies catch rates of historical swept area surveys

The use of different trawl nets is a factor to be considered when analysing historical swept-area research surveys aimed to characterize temporal variations in the relative abundance of demersal resources. If factors that may affect the performance of the bottom trawls were considered, more reliable temporal trends could be established. Due to the high species diversity that characterizes tropical areas, this kind of analysis is often carried out at the multispecies level. Therefore, the objective of this study was to establish the effect of two technical factors, mesh size and towing speed on the multispecies catch rates obtained in different demersal surveys carried out in the Colombian Caribbean Sea between 1988 and 2001, using two generalized linear models: one covering the entire study area and another restricted to one eco-region. For the global model, the effect of the mesh size on the multispecies catch rates was marginally significant (p<0.10), unlike the towing speed, whose effect was not significant (p>0.10). In contrast, for the eco-region model, the effect of mesh size was not significant (p>0.10), while towing speed had a significant effect (p<0.05). Size structure analysis showed escapement mainly through the codend meshes for the larger mesh size evaluated (50.8mm), confirming the appropriateness of considering mesh size when analyzing historical data of swept area surveys. The effect of the towing speed, beyond the clear incidence on the area actually swept, point to the complexity of the relationship between speed and catch rates. In brief, the results showed that, when assessing historical databases, indices of relative abundance of tropical demersal resources can be improved by including mesh size and towing speed factors.

Multi year observations reveal variability in residence of a tropical Demersal Fish, Lethrinus nebulosus: implications for spatial management.

Off the Ningaloo coast of North West Western Australia, Spangled Emperor Lethrinus nebulosus are among the most highly targeted recreational fish species. The Ningaloo Reef Marine Park comprises an area of 4,566 km2 of which 34% is protected from fishing by 18 no-take sanctuary zones ranging in size from 0.08–44.8 km2. To better understand Spangled Emperor movements and the adequacy of sanctuary zones within the Ningaloo Reef Marine Park for this species, 84 Spangled Emperor of a broad spectrum of maturity and sex were tagged using internal acoustic tags in a range of lagoon and reef slope habitats both inside and adjacent to the Mangrove Bay Sanctuary zone. Kernel Utilisation Distribution (KUD) was calculated for 39 resident individuals that were detected for more than 30 days. There was no relationship with fish size and movement or site fidelity. Average home range (95% KUD) for residents was 8.5±0.5 km2 compared to average sanctuary zone size of 30 km2. Calculated home range was stable over time resulting in resident animals tagged inside the sanctuary zone spending ∼80% of time within the sanctuary boundaries. The number of fish remaining within the array of receivers declined steadily over time and after one year more than 60% of tagged fish had moved outside the sanctuary zone and also beyond the 28 km2 array of receivers. Long term monitoring identified the importance of shifting home range and was essential for understanding overall residency within protected areas and also for identifying spawning related movements. This study indicates that despite exhibiting stable and small home ranges over periods of one to two years, more than half the population of spangled emperor move at scales greater than average sanctuary size within the Ningaloo Reef Marine Park.

Length selectivity of commercial fish traps assessed from in situ comparisons with stereo-video: Is there evidence of sampling bias?

Stock assessments of tropical demersal teleost fisheries generally rely on fishery-dependent samples of age structure. Baited remote underwater stereo-video (stereo-BRUV) systems can provide fishery independent information on the biases and selectivity of particular fishing gears in relation to the length distribution of what is potentially available for capture. In the tropical demersal fishery off northwestern Australia the length distributions of 12 species sampled by standard commercial fish traps and stereo-BRUVs were compared using kernel density estimation techniques. We found no significant differences in the shape of the length distributions sampled by either method. However, the location (mean length) of the length distribution of seven of the 11 target species and the one bycatch species differed between the sampling methods. A family-specific trend was exhibited, likely due to the ambush behaviour of larger epinephelids in traps and the saturation of the field of view of stereo-BRUV by schools of smaller lutjanids. However, the difference in mean length was not biologically significant for these families. These results indicate that samples of target species derived from commercial trap catches are likely to provide representative and robust estimates of vital population statistics and life history parameters.

Relationship between pelagic larval duration and abundance of tropical fishes on temperate coasts of Japan

The influence of pelagic larval duration (PLD) and egg type dispersal capabilities of 35 demersal and pelagic-spawning tropical fish species is examined in relation to their abundance on the temperate coasts of Japan. The PLDs of pelagic spawners were significantly longer than those of demersal spawners, and a high occurrence of pelagic spawners on the temperate coasts suggests that these fishes are more easily transported to temperate coasts than demersal spawners. For demersal spawners, the common species on the temperate coasts had significantly longer PLDs than the rare species; this suggests that PLD is a major factor influencing the distribution patterns of tropical demersal spawners on temperate coasts. Moreover, a negative correlation between PLD and the abundance of some species of pelagic and demersal spawners suggests the presence of reproductively active fishes in northern subtropical and even in temperate waters.

Analysis of stable isotope ratios to investigate stock structure of red emperor and Rankin cod in northern Western Australia

The ratios of stable isotopes 18O/16O and 13C/12C, in sagittal otolith carbonate from two tropical demersal teleosts, red emperor Lutjanus sebae and Rankin cod Epinephelis multinotatus, from several locations in northern Western Australia, differed between sites. On a broad scale, fish from the four locations, Shark Bay, Ningaloo, Pilbara, and Broome had stable isotope values that were sufficiently different to indicate separate stocks, and it is appropriate to manage these populations of the two species independently in these areas. On a smaller scale, there may be limited mixing of these species between the Pilbara trawl fishery and the trap and line fisheries operating out of Onslow and Broome. Values of stable oxygen isotopes were strongly related to sea surface temperature, although there were some sites in shallow water where low values of stable oxygen isotopes indicated that fish were living in warm water. The use of stable oxygen and carbon isotope values is a valuable, cost effective method of determining the degree of mixing of fish stocks.

Changes in species composition of demersal fish fauna of Southeast Gulf of Carpentaria, Australia, after 20 years of fishing

The changes in a tropical demersal fish community in the southeast Gulf of Carpentaria, Australia, were examined by comparing the results of a survey undertaken in 1964 before the area was fished, with two surveys in 1985 and 1986 after 20 yr of commercial trawl fishing. The numerical abundance of 52 of the 82 fish taxa had not changed significantly, but that of 18 had decreased while 12 had increased. One taxon (Paramonacanthus spp.) had decreased by more than 500 times and another (Saurida micropectoralis) had increased substantially in abundance. The changes occurred throughout the area surveyed, but the largest changes were offshore. In the pre-trawling community most taxa were caught mainly during the night, while in the post-trawling community most were caught in the same numbers during day and night. In 10 of the 30 taxa that changed in abundance, changes were within a family and could not be explained. For the remaining 20, the changes could be related to their position in the water column: benthic taxa decreased and bentho-pelagic taxa increased. The changes were assessed in relation to fishing effort, and changes in the mud content of the substrate in the study area. Although the changes did not correlate with the fishing effort among three zones in the study area, it is suggested that fishing effort and discarding of the by-catch caused the change in 18 taxa. The magnitude of the decreases of some species might be related to changes in the sediment or possibly other long-term environmental change. There was also a change in the diel behaviour in the fish community that perhaps may be related to the effects of fishing on a tropical multispecies fish community.

Growth rate of tropical demersal fishes

Growth rate of tropical demersal fishes

Life History Strategies and Recent Changes in Population Structure in the Lizardfish Genus, Saurida, on the Australian Northwest Shelf

Aspects of the life history of four sympatric species of the lizardfish genus, Saurida, were examined based on material obtained by demersal trawling on the Northwest Shelf of Australia. The four vary widely in age and size at sexual maturity and in maximum age and length: two species (Saurida undosquamis and Saurida sp. 1) reach large size and live for 6-7 yr, whereas two others (S. longimanus and Saurida sp. 2) reach sexual maturity within a few months of settling to the substratum and may rarely live more than a year. The minimum generation time for Saurida sp. 2 is shorter than that yet reported for any other species of tropical demersal fish species. These life history features are discussed in relation to recent changes in the lizardfish population on the Northwest Shelf and compared to those of other tropical demersal fishes. Limited data suggest that very short generation times may not be unusual for such demersal fishes, which may have important implications for both population dynamics and management of tropical fish species.

Ocean Sciences in Relation to Living Resources

The Intergovernmental Oceanographic Commission's Resolution XI-17 on Ocean Sciences in Support of Living Resources specifies that programs should be formulated in applied ocean research related to fishery problems. In this report we identify some types of needed research. We discuss the relative importance of the various life history stages to the question of resource variation regarding difficulties in sampling or studying on relevant scales. Our report stresses identification and accounting for causal climate-driven ambient variations. Some testable hypotheses relating the larval stages are given as examples of useful research. We also describe research needed on predation and species interactions. Examples of specific systems where important ecological processes need intense study are given. Upwelling areas, coral reefs, tropical and shelf demersal fisheries, and the open ocean fisheries each require different types and scales of study. The importance of data accessability and multidisciplinary research activities, beginning with dialogues between the ocean research and fishery resource scientists is paramount to successful completion of these tasks. The examples provided may stimulate further effort to identify resource research problems and programs for resolving them.Key words: ocean science, living resources, life history cycles, fishery recruitment, predation, species interactions, fish eggs and larvae, vertical stability of the ocean, advection, convergence and divergence, upwelling systems, coral reefs, tropical demersal fisheries

The Exploitation of African Fisheries

Substantial fisheries exist on the major lakes and rivers of Africa. The seas off the western coast of Africa are amongst the most productive in the world but the eastern coasts are relatively poor. Lake Victoria is used as a case study to demonstrate how overfishing of some stocks, such as Sarotherodon esculentus, has occurred, how it could have been predicted by conventional models developed from temperate demersal fisheries and how the situation could be improved. The difficulties of assessing multispecific fisheries, i.e. where the whole community is exploited, are dealt with particularly in relation to proposed trawling operations in the lakes. Conflict between the trawl and artisan fisheries seems inevitable. Some shortcomings of fish culture and stocking programmes are discussed and attention is drawn to the increased activities of dealers in ornamental fish and the emerging problem of pollution. Our ignorance of how river fisheries operate is considerable. Tentative estimates of yield and models of river fisheries are only just beginning to appear. It is evident that the factors involved are very different from those of lake fisheries. Off the western sea board, the coastal states are at a severe disadvantage. Not only are the products of the major industrial fisheries off South Africa and Morocco exported to the developed countries but the majority of the catch from tropical waters is taken by foreign long-range fleets. The difficulties of Ghana in establishing a distant-water fleet for food fish are contrasted with the success of the South Koreans in establishing a world-wide tuna industry mainly for export. The importance of developing the long-range capability to take advantage of the probable general increase in territorial waters is emphasised. Some of the problems associated with management of tropical demersal and pelagic stocks are dealt with and questions are raised as to the approach required to deal with these when development is the priority.