Jackknife method for estimating the variance of the age composition using two-phase sampling with an application to commercial catches of swordfish (Xiphias gladius)

An estimate of the age composition of a commercial swordfish catch and its variance was obtained using the replicate full jackknife estimator for two-phase sampling where the sampling units in the first phase are clusters (vessel-trips) and the units of the second phase are individual fishes. The jackknife variance estimator was compared with an analytical estimator of variance commonly used in fisheries studies which ignores the clustering, and assumes simple random sampling of individual fish. The application of these naïve estimators to the Chilean South Pacific swordfish fishery results in an underestimation of the variance. This is a consequence of using an inappropriate primary sampling unit of analysis in the first-phase sample where the vessel-trip unit is replaced by an individual fish unit, thus ignoring the intra-cluster correlation in age samples. The jackknife estimator of variance is an alternative useful for complex two-phase sampling designs.     

Metazoan parasites on gills of Southern Bluefin Tuna (Thunnus maccoyii) do not rapidly proliferate after transfer to sea cages

A relatively new and highly valuable aquaculture industry focuses on three species of bluefin tunas, which are captured from the wild and fattened for several months in sea cages. In teleost aquaculture, mortalities and extra production costs are very commonly associated with metazoan ectoparasites. In tuna, however, the production value lost due to diseases associated with ectoparasites is unknown. We collected epidemiological data on burdens of metazoans on the gills of farmed southern bluefin, Thunnus maccoyii, in a series of monthly samples of tuna from the time of stocking through to harvest (March to August, 2004; N = 210) in five sea cages on a farm off Port Lincoln, Australia. Three species were recorded; for one (a copepod, Pseudocycnus appendiculatus), there was a gradual, significant increase in both abundance (from a mean of 0.1 in March, to 3.83 in August) and prevalence (from 10% to 67.5%). For the other two species (a second copepod, Euryphorus brachypterus, and a polyopisthocotylean flatworm, Hexostoma thynni) there were no discernible trends in prevalences and abundances. These results contrast markedly with those of other intensively cultured species of finfishes, in which parasite epizootics are frequent. This finding may indicate that despite the stresses of captivity, tuna mount a robust immune response to ectoparasites; the relatively low stocking densities at which tuna are farmed may facilitate this. The fall in water temperature during farming (22 °C to 13 °C) may also reduce the reproductive rate of these ectoparasites.     

Application of bioelectrical impedance analysis as a method for estimating composition and metabolic condition of southern bluefin tuna (Thunnus maccoyii) during conventional tagging

Tagging fish without gathering physiological information may be a wasted opportunity. We tested bioelectrical impedance analysis (BIA) for measurement of relative condition of southern bluefin tuna (Thunnus maccoyii) during conventional tagging at sea. We refined the equipment and method by measurement of 360 fish during conventional and acoustic tagging. Our results demonstrate that BIA is an accurate measure of condition for southern bluefin tuna in the same way it has been shown to be for metabolic condition and composition in other vertebrates including humans. Further, there is sufficient variation in BIA measures of the natural population to give meaningful measures of both metabolic condition and composition between groups at different times and developmental stages. Condition of tuna in this study may be related to the ocean environment just prior to measurement. BIA meets the necessary objectives for measuring fish condition during tagging as it is shown to be harmless, reliable, quick, and effective and does not disrupt conventional tagging operations. In the light of these results this type of condition measurement should be taken wherever possible in future tagging operations for this and other similar species, which will generate new insight into the ecological challenges faced by pelagic fishes. The ability to relate recent ocean environments and subsequent patterns in fish survival may lead to changes in the way tagging data is interpreted.     

Incorporating spatial autocorrelation into the general linear model with an application to the yellowfin tuna (Thunnus albacares) longline CPUE data

Catch-per-unit-effort (CPUE) data have often been used to obtain a relative index of the abundance of a fish stock by standardizing nominal CPUE using various statistical methods. The theory underlying most of these methods assumes the independence of the observed CPUEs. This assumption is invalid for a fish population because of their spatial autocorrelation. To overcome this problem, we incorporated spatial autocorrelation into the standard general linear model (GLM). We also incorporated into it a habitat-based model (HBM), to reflect, more effectively, the vertical distributions of tuna. As a case study, we fitted both the standard-GLM and spatial-GLM (with or without HBM) to the yellowfin tuna CPUE data of the Japanese longline fisheries in the Indian Ocean. Four distance models (Gaussian, exponential, linear and spherical) were examined for spatial autocorrelation. We found that the spatial-GLMs always produced the best goodness-of-fit to the data and gave more realistic estimates of the variances of the parameters, and that HBM-based GLMs always produced better goodness-of-fit to the data than those without. Of the four distance models, the Gaussian model performed the best. The point estimates of the relative indices of the abundance of yellowfin tuna differed slightly between standard and spatial GLMs, while their 95% confidence intervals from the spatial-GLMs were larger than those from the standard-GLM. Therefore, spatial-GLMs yield more robust estimates of the relative indices of the abundance of yellowfin tuna, especially when the nominal CPUEs are strongly spatially autocorrelated.     

Influence of size, age and maternal condition on the oocyte dry weight of Argentine hake (Merluccius hubbsi)

Effects of maternal age–size and condition on oocyte dry weight were analysed for Merluccius hubbsi. Samples of mature females were collected in the Patagonian spawning area during 10 research surveys carried out between 2000 and 2005. Dry weight of 50 hydrated oocytes ranged from 0.80 to 1.90 mg. Relationships of this variable with total length and age were best described by a logarithmic model during the spawning peak. At the beginning and at the end of the reproductive season no significant relationship between these variables was observed, but a decrease in egg dry weight was detected during the spawning period. In general, young spawners (≤5-year-old) showed more variability in egg dry weight, which was associated with a reproductive season shorter than that of old females. Relative condition factor did not shows a significant relationship with oocyte dry weight, and mean values of this index were similar for different months of the spawning seasons analysed.     

Acoustic telemetry versus monitored longline fishing for studying the vertical distribution of pelagic fish: bigeye tuna (Thunnus obesus) in French Polynesia

This study compares detailed, nearly continuous, observations on bigeye tuna, Thunnus obesus equipped with electronic tags, with discrete observations on a larger number of individuals from fishing experiments in order to validate the use of instrumented longlines to study the vertical distribution of fish. We show that the depth distributions obtained from the two different observation techniques regarding different environmental variables (temperature, dissolved oxygen (DO), prey distribution) are similar. Bigeye tuna do not seem to be attracted by baits in the vertical dimension (no modification of their vertical distribution by the fishing gear), which allows the use of instrumented longlines to study the vertical behaviour of pelagic species. This technique, when used with appropriate deployment strategy, could therefore represent an alternative to electronic tags (acoustic or archival tags) when there is a need to determine the vertical distribution of fish species by size or sex, in different environments for the study of fishery interactions.     

Comparison of age estimates from scale, opercular bone, otolith, vertebrae and dorsal fin ray in Labeo rohita (Hamilton), Catla catla (Hamilton) and Channa marulius (Hamilton)

The present study was undertaken with a view to compare the precision and reliability of the age readings obtained from different bony structures of some important freshwater teleosts viz., Labeo rohita (Hamilton), Catla catla (Hamilton) and Channa marulius (Hamilton). Standard procedures were followed to prepare and study the age structures. In L. rohita and C. marulius percent agreement between reader's age estimates was highest for scales, i.e. 96.3% and 90.5%, respectively and in C. catla percent agreement was highest (93.3%) for opercular bone. When scale ages were compared with other alternative structures viz., otoliths, opercular bone, vertebral centra and dorsal fin rays, percent agreement was found highest between scale and opercular bone age estimates (77.8%) in L. rohita and between scale and otoliths (94.8%) in C. marulius. In case of C. catla highest percent agreement was found between opercular bone and scale age estimates. In L. rohita each of the ageing structure showed significant (P < 0.05) underestimation of age in comparison to scales. In C. catla mean age estimates from opercular bone were comparable (P > 0.05) to the values obtained from all other structures except dorsal fin rays. In C. marulius mean age estimates from scales were comparable (P > 0.05) to those from all other structures except from dorsal fin rays. Results indicated scales to be the most suitable structure for ageing L. rohita and C. marulius and opercular bone for C. catla. However, in C. catla also scales may be used as a non-destructive method of age estimation with satisfactory results.     

Comparison of methods for sampling populations of ghost shrimp, Trypaea australiensis (Decapoda: Thalassinidea: Callianassidae)

Coring and pumping methods, commonly used to determine the abundance and population parameters of burrowing ghost shrimp, were assessed in three widely separated south-eastern Australian estuaries. First, a comparison of two sizes of coring tube (0.04 and 0.07 m²) revealed no significant differences in catches of the ghost shrimp Trypaea australiensis Dana, nor differences in precision. However, the smaller sized core was consistently more cost effective. Second, when sampling a 0.1 m² quadrat, it was found that 12 “pumps” of a suction pump produced significantly higher numbers of individuals at most sites than using either four, eight or 16 pumps. In a third set of experiments, the coring method was compared to the pumping method, in terms of sampling efficiency, abundance estimates and sex ratios of captured ghost shrimp. Pumping was found to be more efficient, catching significantly greater numbers of ghost shrimp per minute than the coring. In addition, pump catches displayed a more normal size distribution than catches from the core and included a wider range of sizes. Sex ratios of catches from coring was 1:1, while catches from pumping were significantly biased toward females (0.71:1). Since female-biased sex ratios are characteristic of many thalassinid populations, this latter result probably reflects actual sex ratios for T. australiensis and not method based size selectivity.     

鱼类标志放流步骤的优选及其在黄鳍棘鲷中的应用

鱼类标志放流过程中,关键细节缺失参考依据易导致标志鱼因标志操作不规范而死亡(或导致标志脱落),从而影响基于标志群体抽样的增殖效果评估、放流群体时空格局等后续研究的准确性。本研究以南海重要增殖放流鱼类黄鳍棘鲷为对象,采用多因素方差分析对比了标志过程中关键操作(标志前麻醉与否、标志部位、植入角度)的生长率、存活率、标志保留率的差异。40 d的实验结果显示,不同标志操作对鱼的生长无显著影响。麻醉与否对实验鱼的存活率影响极显著。标志部位、植入角度对标志保留率影响显著。优选出的最佳标志操作组合为麻醉,将T型标志以45°植入背鳍基前部肌肉(存活率95.56%、标志保留率98.89%)。综合以往资料,本研究提出了黄鳍棘鲷[体长(10.05±0.39)cm]T型标志操作规范建议,为今后科学开展标志放流提供参考依据:①标志前暂养,将待标志鱼放入培育池内暂养3 d或以上,标志前24 h停食;②材料消毒,将T型标志和标志枪针头用75%酒精浸泡消毒5 min;③麻醉,用30 mg/L丁香酚溶液(或MS-222麻醉剂)麻醉至鱼体腹部向上翻转时,迅速进行标志;④标志,用标志枪针头拨去标志部位的1个鳞片,然后针头与鱼体呈45°将T型标志植入背鳍基前部肌肉;⑤鱼体消毒,将标志鱼放入含有5%聚维酮碘(或高锰酸钾)的海水溶液中药浴消毒30 min;⑥标志后暂养,消毒后的标志鱼人工暂养7 d后可放流。     

Geographic influences on and the accuracy and precision of age estimates for the red bass, Lutjanus bohar (Forsskal 1775): A large tropical reef fish

The red bass is a large tropical reef fish (Lutjanidae, tropical snappers) that is harvested to varying extents throughout a widespread Indo-Pacific distribution. The aims of this study were to investigate the accuracy and precision of age estimates from transverse sections of sagittal otoliths and to assess effects on these of the geographic area of collection and otolith preparation method. Two independent validation studies suggested an approximately annual formation of annuli in otoliths, predominantly for otoliths with 4–10 annuli but also for one otolith with 29 annuli. Otolith sections produced exceptionally high annulus counts: up to 56 annuli for samples from the Great Barrier Reef (GBR), Australia; and up to 55 annuli from the Seychelles, indicating a high longevity for this species. The precision of otolith readings from the GBR (index of average percent error, IAPE = 3.21 ± 0.26 S.E.) was within commonly accepted bounds for age estimation (IAPE up to 5%) but precision of readings from the Seychelles was significantly lower (IAPE = 9.18 ± 0.47 S.E.) and outside of this “acceptable” range. Age-based biological parameters for red bass from the Seychelles should thus be applied with greater caution than those for red bass from the GBR. If basic demographic properties are assumed relatively constant across the wide geographic range sampled, however, results from the GBR could be used as more reliable preliminary data for precautionary management strategies in the Seychelles and elsewhere.