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.

Using an inverse-logistic model to describe growth increments of blacklip abalone (Haliotis rubra) in Tasmania

A new description of growth in blacklip abalone (Haliotis rubra) with the use of an inverse-logistic model is introduced. The inverse-logistic model avoids the disadvantageous assumptions of either rapid or slow growth for small and juvenile individuals implied by the von Bertalanffy and Gompertz growth models, respectively, and allows for indeterminate growth where necessary. An inverse-logistic model was used to estimate the expected mean growth increment for different black-lip abalone populations around southern Tasmania, Australia. Estimates of the time needed for abalone to grow from settlement until recruitment (at 138 mm shell length) into the fishery varied from eight to nine years. The variability of the residuals about the predicted mean growth increments was described with either a second inverse-logistic relationship (standard deviation vs. initial length) or by a power relationship (standard deviation vs. predicted growth increment). The inverse-logistic model can describe linear growth of small and juvenile abalone (as observed in Tasmania), as well as a spectrum of growth possibilities, from determinate to indeterminate growth (a spectrum that would lead to a spread of maximum lengths).

Integrating fish into seasonally flooded rice fields: on-farm trials in Assam, India

In the State of Assam, floodplains cover 2.6 million ha of area that is traditionally rice growing. The ecosystem in the rice-growing areas has undergone major changes as a result of various developmental activities and adoption of modern farming technology. Rice fields were once the major source of fish for the rural farmers. There has been a sharp decline in fish population in rice field leading to a chronic shortage of fish in the State and a deterioration of the rice ecosystem. This paper describes two on-farm experiments for integrating rice cultivation and fish production with the intent of contributing to the understanding of how raising fish can improve rice yields, riceecosystems and farm incomes.

Integrating fish and azolla into rice-duck farming in Asia

Several countries in Asia practice integrated rice-duck farming. On-farm resources such as duck manure and feed waste are not adequately used and recycled in the system. This indicates the potential for research to increase the productivity of the rice-duck system. The integration of fish and the nitrogen-fixing aquatic fern azolla show promise for increasing the production potential of the system. Fish, azolla and ducks integrated with rice farming can result in nutrient enhancement, pest control, feed supplementation and biological control. Some of the results of a case study on integrated rice-fish-azolla-duck farming systems conducted in the Philippines are presented in this paper.