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

Prevalence of pelagic dependence among coral reef predators across an atoll seascape

1)Coral reef food webs are complex, vary spatially and remain poorly understood. Certain large predators, notably sharks, are subsidised by pelagic production on outer reef slopes, but how widespread this dependence is across all teleost fishery target species and within atolls is unclear. 2)North Malé Atoll (Maldives) includes oceanic barrier as well as lagoonal reefs. Nine fishery target predators constituting ca. 55% of the local fishery target species biomass at assumed trophic levels 3‐5 were selected for analysis. Data were derived from carbon (δ13C), nitrogen (δ15N) and sulfur (δ34S) stable isotopes from predator white dorsal muscle samples, and primary consumer species representing production source end‐members. 3)Three‐source Bayesian stable isotope mixing models showed that uptake of pelagic production extends throughout the atoll, with predatory fishes showing equal planktonic reliance between inner and outer edge reefs. Median plankton contribution was 65‐80% for all groupers and 68‐88% for an emperor, a jack and snappers. 4)Lagoonal and atoll edge predators are equally at risk from anthropogenic and climate‐induced changes which may impact the linkages they construct, highlighting the need for management plans that transcend the boundaries of this threatened ecosystem

Movements and habitat use by the endangered Australian freshwater Mary River cod, Maccullochella peelii mariensis

The decline of the endangered Mary River cod, Maccullochella peelii mariensis, of Queensland, Australia, has been attributed to anthropomorphic habitat alterations, however, the relationships between this subspecies and its physical environment are poorly understood. We used radiotelemetry to investigate the movements and use of habitats by nine Mary River cod (total length 420–760 mm; weight 1.3–5.5 kg) in the Mary River system over a 20 month period. The mean distance moved each month was positively correlated with monthly stream discharge, but the direction of movements was unpredictable. Patterns of movement varied considerably among individuals and appeared to be unrelated to size. Cod established home ranges of between 70 and 820 m in length. Five of the tagged cod returned to a previous home range after moving more than 10 km, whereas three did not move more than 2 km from their capture location for the duration of the study. Cod used large woody debris complexes more than any other type of habitat, and rarely used areas of open water. Managers should give high priority to maintenance of fish passage and protection/rehabilitation of large woody debris habitats if cod populations are to recover

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