Integrating methods for determining length-at-age to improve growth estimates for two large scombrids

Fish growth is commonly estimated from length-at-age data obtained from otoliths. There are several techniques for estimating length-at-age from otoliths including 1) direct observed counts of annual increments; 2) age adjustment based on a categorization of otolith margins; 3) age adjustment based on known periods of spawning and annuli formation; 4) back-calculation to all annuli, and 5) back-calculation to the last annulus only. In this study we compared growth estimates (von Bertalanffy growth functions) obtained from the above five methods for estimating length-at-age from otoliths for two large scombrids: narrow-barred Spanish mackerel (Scomberomorus commerson) and broad-barred king mackerel (Scomberomorus semifasciatus). Likelihood ratio tests revealed that the largest differences in growth occurred between the back-calculation methods and the observed and adjusted methods for both species of mackerel. The pattern, however, was more pronounced for S. commerson than for S. semifasciatus, because of the pronounced effect of gear selectivity demonstrated for S. commerson. We propose a method of substituting length-at-age data from observed or adjusted methods with back-calculated length-at-age data to provide more appropriate estimates of population growth than those obtained with the individual methods alone, particularly when faster growing young fish are disproportionately selected for. Substitution of observed or adjusted length-at-age data with back-calculated length-at-age data provided more realistic estimates of length for younger ages than observed or adjusted methods as well as more realistic estimates of mean maximum length than those derived from backcalculation methods alone

Big fish in shallow water; use of an intertidal surf-zone habitat by large-bodied teleosts and elasmobranchs in tropical northern Australia

Shallow intertidal habitats are recognised as critical for larval and juvenile fish, and are often assumed to function as refuge areas where predation risk is reduced. Yet there is growing evidence that suggest these areas may also be regularly inhabited by large bodied fish and be the site of high levels of juvenile fish predation. In the present study we examined the use of an intermittently available surf zone habitat in tropical northern Australia by a diverse community of large-bodied teleosts (mean total length 444 mm), sharks (mean total stretched length 658 mm) and rays (mean total stretched length 1,108 mm). Drawing on the methods and ecological knowledge of a local commercial fishery, gillnets were used to capture fish as they entered the surf zone on the flooding tide. Monthly surveys over a one-year period revealed a dynamic assemblage consisting of 30 species of teleosts (mostly caught as adults) and 14 species of sharks and rays (mostly caught as juveniles or young of the year). Although it is unclear why these fish use this habitat, we conclude that it may support a broad range of biological benefits including spawning for teleosts, parturition for elasmobranchs, as well as foraging and refuge. The unique findings of this study highlight existing knowledge gaps and the need to better understand what fish use the intertidal zone and why. This should be a high priority given the increasing anthropogenic pressures on coastal margins

Growth trends of Queensland east coast Spanish mackerel (Scomberomorus commerson) from otolith back-calculations

The growth of narrow-barred Spanish mackerel (Scomberomorus commerson) was examined over a broad spatial scale encompassing the east coast of Queensland, Australia. Our aim was to use back-calculation to identify potential biases in growth estimation, and to provide a more recent estimate of growth for Spanish mackerel on the east coast of Queensland. Growth trends were determined using the body proportional hypothesis (BPH) back-calculation model and compared between back-calculated and observed length-at-age data. The back-calculated von Bertalanffy growth function parameters estimated for males and females, respectively, were L∞ = 1047 mm, K = 0.75, t0 = –0.18 and L∞ = 1248 mm, K = 0.51 and t0 = –0.39. Significant differences in growth of Spanish mackerel sampled among different regions along the east coast were observed. A comparison of back-calculated and observed length-at-age data revealed significant biases in the observed length-at-age data for younger aged fish. Our findings contrast with previous estimates of growth for Spanish mackerel from the east coast of Queensland and highlight the use of back-calculation for the first few annuli as a means of avoiding biases in growth estimation arising from gear selectivity associated with fishery-dependent samples of fast-growing fish

Effect of changes in hook pattern and size on catch rate, hooking location, injury and bleeding for a number of tropical reef fish species

The Queensland Great Barrier Reef line fishery in Australia is regulated via a range of input and output controls including minimum size limits, daily catch limits and commercial catch quotas. As a result of these measures a substantial proportion of the catch is released or discarded. The fate of these released fish is uncertain, but hook-related mortality can potentially be decreased by using hooks that reduce the rates of injury, bleeding and deep hooking. There is also the potential to reduce the capture of non-target species though gear selectivity. A total of 1053 individual fish representing five target species and three non-target species were caught using six hook types including three hook patterns (non-offset circle, J and offset circle), each in two sizes (small 4/0 or 5/0 and large 8/0). Catch rates for each of the hook patterns and sizes varied between species with no consistent results for target or non-target species. When data for all of the fish species were aggregated there was a trend for larger hooks, J hooks and offset circle hooks to cause a greater number of injuries. Using larger hooks was more likely to result in bleeding, although this trend was not statistically significant. Larger hooks were also more likely to foul-hook fish or hook fish in the eye. There was a reduction in the rates of injuries and bleeding for both target and non-target species when using the smaller hook sizes. For a number of species included in our study the incidence of deep hooking decreased when using non-offset circle hooks, however, these results were not consistent for all species. Our results highlight the variability in hook performance across a range of tropical demersal finfish species. The most obvious conservation benefits for both target and non-target species arise from using smaller sized hooks and non-offset circle hooks. Fishers should be encouraged to use these hook configurations to reduce the potential for post-release mortality of released fish

The post-release behaviour and fate of tournament-caught smallmouth bass after ‘fizzing’ to alleviate distended swim bladders

In recreational fishing, barotrauma occurs when fish that are angled from deep water are rapidly brought to the surface, causing a number of injuries and physiological alterations associated with gas expansion (such as distended swim bladders) that may impede swimming and prevent the fish from returning to depth. By deflating the swim bladder using a hypodermic needle (a process called “fizzing”) fish typically can return to depth. However, little is known about its consequences and efficacy on wild fish. At a competitive smallmouth bass (Micropterus dolomieu) angling event on Rainy Lake in northwestern Ontario, we evaluated the effects of post-weigh in fizzing on the behaviour and short-term survival of three treatment groups after release: (i) barotrauma fish without fizzing (N = 8); (ii) barotrauma fish that were fizzed (N = 9); (iii) fish without signs of barotrauma that were fizzed (N = 10) (sham control). Small external radio transmitters were affixed to the fish and tracked for 4 days. Fish were released at a common site and we assessed their dispersion at specific distances from the release site (50, 250, and 2000 m). All fish survived the 4-day monitoring period. No differences were observed in the time it took each group to disperse from the release site. Furthermore, there was no statistical evidence that fizzing influenced mean daily movements relative to controls, though a consistent trend was noted where fish that were fizzed displayed greater movement than non-fizzed fish with distended swim bladders. This study revealed that fizzing by trained experts is not detrimental to barotrauma fish. However, if done improperly there is risk to vital organs suggesting that there is merit in exploring other less invasive approaches to recompressing fish. Because our statistical power was generally low, further research is needed to determine whether fizzing should be encouraged or dissuaded to maintain the welfare status of the fish, decrease sublethal impairments, and reduce mortality

Movement patterns of two carangid species in inshore habitats characterised using network analysis

Carangids are important ecological components of coastal and reef habitats, in addition to their economic significance as a target species for some fishers. Despite these important ecological and economic roles, little information is available on the movement ecology of these species. Passive acoustic monitoring was used to track the movements of 16 giant trevally Caranx ignobilis and 20 golden trevally Gnathanodon speciosus in Cleveland Bay off the coast of Queensland, Australia. Long-term observations of behaviour and movement were recorded via a network of acoustic receivers, and a network analysis approach (a novel, alternative approach to conventional movement analysis) was applied to the collected data. Tagged individuals were present in the study region between 30 and 394 d (mean +/- SD: 166 +/- 116 d) with a mean +/- SE residency index of 0.7 +/- 0.1. Notable inter-annual variation occurred with individuals that were detected on more days and more receivers, moved more frequently, and were more resident in some years than in others. In addition, movement patterns differed between species, with C. ignobilis being detected on fewer days by fewer receivers and moving less than G. speciosus. Network analysis revealed that a combination of factors including ontogeny, foraging niche, and habitat influences may explain differences in space use between the species. These results highlight unique behaviours between co-occurring and closely related species and enhance our understanding of animal interactions in inshore habitats

Evaluating catch and mitigating risk in a multispecies, tropical, inshore shark fishery within the Great Barrier Reef World Heritage Area

Small-scale and artisanal fisheries for sharks exist in most inshore, tropical regions of the world. Although often important in terms of food security, their low value and inherent complexity provides an imposing hurdle to sustainable management. An observer survey of a small-scale commercial gill-net fishery operating within the Great Barrier Reef World Heritage area revealed at least 38 species of elasmobranch were present in the catch. Of the total elasmobranch catch, 95% was 25 species of Carcharhiniformes from the families Carcharhinidae, Hemigaleidae and Sphyrnidae. Individual species were captured in a variety of ways by the fishery, often with strongly biased sex ratios and in a variety of life stages (e.g. neonates, juveniles, adult). Despite this, the main carcharhiniform taxa captured could be qualitatively categorised into four groups based on similar catch characteristics, body size and similarities in life history: small coastal (<1000 mm); medium coastal (1000–2000 mm); large coastal/semi-pelagic (>2000 mm); and hammerheads. Such groupings can potentially be useful for simplifying management of complex multispecies fisheries. The idiosyncrasies of elasmobranch populations and how fisheries interact with them provide a challenge for management but, if properly understood, potentially offer underutilised options for designing management strategies

Accepting final counts from repeat readings of otoliths: should a common criterion apply to the age estimation of fish?

Multiple readings of otoliths are often carried out to assess the repeatability and reliability of increment counts for estimating fish age. Various criteria have been used to assign or discard age estimates from repeated counts when discrepancies occur although the reasons for doing so are usually not stated or justified. Trends in relative frequencies (percentage disagreement, PD) and magnitudes (inter-read discrepancy, IRD) of otolith-count discrepancies were explored for 15 species of fish collected from a range of locations around Australia to explore generality in the best explanatory model(s) for otolith-count discrepancies and, hence, the most appropriate criterion for accepting or rejecting age estimates from multiple-count data. Increasing discrepancies with increasing age, according to a constant per-increment probability of error, was the best-approximating model for 9 of the 15 species for PD data but for only two species for IRD data. Our results indicated disproportionately higher rates of rejection of estimates from older age groups if exact agreement between repeated counts was required for age acceptance. Results varied with the reader, region and the method of otolith reading, indicating that multiple criteria for accepting or rejecting counts from multiple readings may be required among or even within species