Horse mackerel fecundity in relation to lipid content

During the 2004 mackerel and horse mackerel triennial egg survey fecundity of horse mackerel (Trachurus trachurus L.) was estimated using a new gravimetric and image analysis system. The fecundity estimates and mean oocyte diameters showed great variation between and within different institutes. Horse mackerel fecundity increased during the spawning season and showed no decline at the end of the spawning season. This indicates that this species is likely to be an indeterminate spawner. The lipid content was estimated from whole horse mackerel collected during the three months prior and during the spawning season. Lipid content rapidly decreased during the period of ovary development prior to the onset of spawning, but remained rather constant during spawning. A relationship between fecundity and lipid content could not be found. A time series of lipid content estimates collected annually or triennially might provide an index for changes in fecundity over time. Sampling needs to be carried out prior to the onset of spawning, when fat content is at its highest. Annual stomach content estimates collected in successive years might provide an index for food availability and feeding success. The stomach analysis carried out in 2004 indicated that horse mackerel was feeding only at a low level during the spawning perio

Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice

A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed

Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice

A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed

Thermal dynamics of ovarian maturation in Atlantic cod (Gadus Morhua)

The timing and success of spawning in marine fish are of fundamental importance to population persistence and distribution and, for commercial species, sustainability. Their physiological processes of reproduction are regulated, in part, by water temperature, and therefore changes in marine climate may have dramatic effects on spawning performance. Using adult Atlantic cod (Gadus morhua) as a case study, we examined the links between water temperature, body size, vitellogenesis, and spawning time by conducting extensive laboratory and field studies. Our experiments documented that vitellogenesis generally starts at autumnal equinox and that oocyte growth and investment are greater in cod held at warmer temperatures. Furthermore, spawning occurred earlier when oocyte growth was more rapid. Large females spawned earlier than smaller females at warmer temperatures, but this effect vanished at colder temperatures. The experimental results were confirmed by measurements of oocyte growth collected from wild-caught cod in northern (Barents Sea) and southern (Irish Sea and North Sea) populations. The established, general model of oocyte maturation was consistent with published egg production curves of cod from these waters, considering relevant in situ temperatures recorded by individual data-storage tags on cod. These findings have considerable relevance for future studies of fish recruitment in relation to climate change

Horse mackerel fecundity in relation to lipid content

During the 2004 mackerel and horse mackerel triennial egg survey fecundity of horse mackerel (Trachurus trachurus L.) was estimated using a new gravimetric and image analysis system. The fecundity estimates and mean oocyte diameters showed great variation between and within different institutes. Horse mackerel fecundity increased during the spawning season and showed no decline at the end of the spawning season. This indicates that this species is likely to be an indeterminate spawner. The lipid content was estimated from whole horse mackerel collected during the three months prior and during the spawning season. Lipid content rapidly decreased during the period of ovary development prior to the onset of spawning, but remained rather constant during spawning. A relationship between fecundity and lipid content could not be found. A time series of lipid content estimates collected annually or triennially might provide an index for changes in fecundity over time. Sampling needs to be carried out prior to the onset of spawning, when fat content is at its highest. Annual stomach content estimates collected in successive years might provide an index for food availability and feeding success. The stomach analysis carried out in 2004 indicated that horse mackerel was feeding only at a low level during the spawning perio

Where do egg production methods for estimating fish biomass go from here?

The special theme volume of Fisheries Research is intended to synthesise the current understanding of the methods and applicability of egg production methods (EPM). It originates from a workshop in Athens which also focused on the future challenges to both the science and logistics of carrying out and using egg production methods. This synthesis addresses three interlinked challenges for those using EPM; how methods have, and need to be, improved, what added value can EPM provide directly to aid advice for management of the marine environment and lastly what extra understanding can EPM bring to marine science? EPM surveys offer some of the most intensive sampling of plankton and adult fish populations in fisheries science. They provide, and will probably provide further insights into fish reproductive processes, embryonic development and spatial and temporal variability in fish populations. Researchers should be encouraged to examine new methods for representative real-time sampling, swift processing of samples and integration of sampling of adults and plankton. EPM provides managers with many “added value” products on habitats and spawning and already provides platforms for monitoring hydrography, zooplankton distributions and acoustic back scatter. Some EPM surveys also incorporate monitoring of birds and sea mammals. EPM, together with aquaculture, has progressed understanding of fish reproductive biology and embryo development. EPM provides long time series of both the ichthyoplankton and fish reproductive traits thus enabling informed study of regime change, variability and ecosystem status. As the EPM become more developed, we expect that these contributions to marine science will increas

Egg production methdos in marine fisheries: An introduction

This paper is an introduction of the Fisheries Research special issue on egg production methods (EPM) that emerged from a dedicated workshop held in Athens, Greece, in 2010. The workshop considered if EPMs are still valid today, it reviewed recent developments in the methods and discussed the utility of EPMs in the future. Importantly, experts from both the daily and the annual egg production methods took part in the workshop. This introduction provides the main concepts underlying EPMs. It also gives a brief history of EPM development over the last two decades with examples of their use worldwide. It provides a review on whether EPMs have fulfilled their objectives. Given their increased utilisation on new fish stocks, and their contribution to scientific advances, EPMs continue to be useful. However, concerns about the bias and precision of the methods remain, and a continuous effort to improve those issues together with the cost-efficiency of the methods is still required