The following two hypotheses, using several years of data collected for Barents Sea and Irish Sea cod (Gad us morhua), are scrutinized: (1) potential fecundity closely reflects realised fecundity, and (2) that realised fecundity or associated properties such as spawning duration or egg weight are not dependent on maternal size or condition. Pre-spawning relative fecundity was dependent on stock, fish length and declined by 27 and 30% in Barents Sea and Irish Sea cod, respectively, due to atresia during maturation from 500 to 750 pm mean follicle diameter. A further decline of 4% in the relative fecundity was found in Barents Sea cod when their relative condition was low. No significant change in the standard deviation of follicle diameter during maturation from 500 to 750 pm could be demonstrated, but fish length and condition were positively correlated. We consider that the standard deviation of vitellogenic follicles moderates an individual fish's spawning duration following experiments with captive Barents Sea cod where spawning duration increased with fish length (5 and 11 days in 52.5 and 96 cm females, respectively at 8.8 degrees C). For the first time we report on spawning atresia where all components of the process are included in the estimate. In Irish Sea cod potential fecundity would make a close approximation (97-98%) to realised fecundity but this assumption was not valid for Barents Sea cod where spawning atresia accounted for a 14-18% fall in relative fecundity depending on the fish length. Batch fecundity was positively related to fish weight and declined as spawning progressed in both stocks especially in the Barents Sea but addition of fish length to the model was not significant. Relative condition declined in both stocks during spawning at the same rate but dropped by a greater proportion in Irish Sea cod. Individual egg dry weight was related to fish length and also declined during spawning. Overall fish size significantly affects some aspects of realised fecundity. (C) 2012 Elsevier B.V. All rights reserved.

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.

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 increase.

In this paper we report on the fate of vitellogenic follicles (VF) as either alpha atretic follicles (αF) or post-ovulatory follicles (POFs) using histology and captive Atlantic cod (Gadus morhua) in three experiments. In Experiment 1 the production and persistence of αF was determined by taking repeated biopsy samples from tagged females held in temperature regimes (mean±SD) controlled at 4.5 (0.3) and 8.1 (0.3)°C. The αF lasted (mean±2 SE, n) 5.3 days (2.5, 8) and 9.7 days (4.9, 8) in the warmer and cooler water respectively and the combined average was 7.5 days (2.9, 16).

Estimation of individual egg production (realized fecundity) is a key step either to understand the stock and recruit relationship or to carry out fisheries-independent assessment of spawning stock biomass using egg production methods. Many fish are highly fecund and their ovaries may weigh over a kilogram; therefore the work time can be consuming and require large quantities of toxic fixative. Recently it has been shown for Atlantic cod (Gadus morhua) that image analysis can automate fecundity determination using a power equation that links follicles per gram ovary to the mean vitellogenic follicular diameter (the autodiametric method). In this article we demonstrate the precision of the autodiametric method applied to a range of species with different spawning strategies during maturation and spawning. A new method using a solid displacement pipette to remove quantitative fecundity samples (25, 50, 100, and 200 milligram [mg]) is evaluated, as are the underlying assumptions to effectively fix and subsample the ovary. Finally, we demonstrate the interpretation of dispersed formaldehyde-fixed ovarian samples (whole mounts) to assess the presence of atretic and postovulatory follicles to replace labor intensive histology. These results can be used to estimate down regulation (production of atretic follicles) of fecundity during maturation.

The effect of controlling food intake during the autumn, which is the time of late vitellogenesis, on fecundity, atresia and follicle and ovary growth was examined for plaice Pleuronectes platessa. Eighteen fish were kept in individual pens and either fed on a high or low ration diet. Fish which increased in whole body condition exhibited an increase in carcass condition which means that when food intake is sufficient to maintain whole body condition some resources are used as storage. Follicle growth rate was positively correlated with change in Fulton’s condition and total atresia was negatively correlated with change in Fulton’s condition. Thus, the rate of vitellogenesis was dependent on the availability of an exogenous food source. Fecundity at the end of the experiment was positively correlated with mass and total length. Food intake had no effect on relative fecundity; however, fish which had a lower food intake lost mass and had a greater intensity of atresia, lowering their absolute fecundity. One fish in a very low condition at the start of the experiment skipped spawning and one fish exhibited a decrease in average follicle diameter during the experiment which is hypothesized to be a prelude to mass atresia.

Flatfish and groundfish show many similarities in reproductive strategies and tactics, both in types present and in responses to fishing pressure or changes in their environment. Over the last 20-30 years the reproduction of Atlantic cod Gadus morhua, Atlantic halibut Hippoglossus hippoglossus, plaice Pleuronectes platessa, sole Solea solea, and turbot Scophthalmus maximus have been extensively studied in the North Atlantic. For cod, halibut and turbot, the research has progressed rapidly due to interest from the aquaculture industry. Extensive overexploitation over many years in combination with climate change represents a potential evolutionary pressure towards changes in growth, lower age at maturity, increased fecundity, smaller egg size (and thereby larval size) and change in spawning time. Early sexual maturity/precocious maturation is also seen in aquaculture and is problematic economically due to a reduction in fillet production. In this paper information is reviewed from studies on both wild and captive populations in experiments, the latter considered important because overexploitation, such as observed in the North Sea, often reduces the natural dynamics in growth and reproduction and complicates collection of sufficiently large samples. Evidence from laboratory experiments demonstrates the inherent plasticity of fecundity production and how this is controlled by food availability and length of photoperiod, while recent information from field studies demonstrates the evolution of genotypes in response to fishing mortality. Today several laboratories have adopted modem techniques for analysis of reproductive investments (fecundity, atresia and sperm characterisation) in controlled experimental situations to explore the effect of temperature or other environmental parameters (such as salinity) on reproduction. These developments, in combination with the rapid implementation of molecular techniques, should make it possible in the future to present highly precise information on reproductive potential, both at the individual and stock level. Of particular interest, and a major goal, would be to dissociate genetic and phenotypic control of reproductive traits arising from a better understanding of gene expression in captive populations. (c) 2007 Elsevier B.V. All rights reserved.

The fecundity of European plaice (Pleuronectes platessa) in the Irish Sea between 2000 and 2004 was estimated during the spawning season for fish in the three main spawning areas (Liverpool Bay, the Cumbrian coast, and the western Irish Sea) and one small spawning group on the west coast of the Isle of Man. Fecundity was also estimated during September of 2003 and 2004. The aim of this was to assess the variability in fecundity between areas and years in the Irish Sea and also to identify when differences in fecundity become apparent in the maturation cycle. There were variations in fecundity on both the temporal and spatial scales. The greatest variation in fecundity between years occurred in the western Irish Sea, whereas there was no variation between years in the southeastern Irish Sea (Liverpool Bay). There was no difference in fecundity between areas or years during September. The maximum fecundity in plaice is determined by the total weight of the fish at the end of follicle recruitment in the ovary, and differences in the fecundity of each population are the result of different levels of down-regulation in the period between the end of follicle proliferation and spawning.

An ELISA for cod vitellogenin (VTG) has been set up using cod lipovitellin for plate coating and standardisation. The assay has been applied to plasma samples collected from male and female cod caught in three distinct areas around the UK, three areas off the Norwegian coast and also to cod reared initially at an aquaculture site and subsequently maintained at a research station. The aim of the study was to determine whether there were any signs of oestrogenic endocrine disruption in a fish species living offshore. VTG induction was found in male cod caught in the North Sea, the Shetland Box area, in Oslofjord and also in cultivated fish. There was a strong relationship between concentrations of VTG and fish size. There was no evidence that the presence of VTG in the plasma of males is a natural part of their life cycle. On the other hand, the size of fish at which these elevated VTG concentrations appear (ca. 5 kg) is about the size that cod change from feeding primarily on benthic invertebrates to mainly other fish, both benthic and pelagic. The possibility is suggested that large cod pick up oestrogenic endocrine disrupters through the food chain.

Apoptosis in vitellogenic, atretic and post ovulatory follicles was studied in Solea solea and Gadus morhua by either measuring 180–200 bp oligonucleotides or by TUNEL in relation to each follicle type found in histological section. Specifically stained apoptotic nuclei were only present in the granulosa layer of recently produced cod post ovulatory follicles and 180–200 bp oligonucleotides were only found in spent sole ovaries.

The spawning biomass of cod (Gadus morhua), plaice (Pleuronectes platessa) and sole (Solea solea) in the Irish Sea in 1995 was estimated by means of the annual egg production method (AEPM). The area surveyed corresponded to ICES assessment area VIIa. This paper describes the sampling design, methods of analysis, estimates of biomass. and sources of error in the estimates. Estimates are also presented for spatially separated spawning grounds of cod and plaice in the eastern and western Irish Sea, the first time that information at this scale has been available. The AEPM estimates of spawning biomass of cod. sole and plaice exceeded the corresponding estimates from Virtual Population Analysis of data from commercial fisheries and trawl surveys by factors of 2.3,2.7 and 4.3 respectively. The sources of these large discrepancies are not yet resolved.

Exposure of adolescent turbot Scophthalmus maximus to low rations during vitellogenesis, covering the 4 months immediately prior to spawning, led to a drop of 70% in mean ovary weight, and was associated with poor growth of the vitellogenic oocytes or, in a third of cases, the absence of vitellogenic oocytes. Exposure to low rations during the recruitment of vitellogenic oocytes, 4–8 months prior to spawning, produced a more variable response. A few of these females produced well‐developed vitellogenic oocytes, but in most, vitellogenic oocytes were absent or only poorly developed and in some cases there was a high incidence of atresia. Exposure to intermediate rations throughout oocyte recritment and vitellogenesis also depressed oocyte development. In maturing females the number of non‐atretic vitellogenic oocytes (relative potential fecundity) averaged 998 per g of somatic body weight of fish at the start of the spawning season in mid‐June. Maturation of the males was not significantly influenced by the dietary regime. The small size of the testes and the low levels of milt production, even in fish fed high rations immediately prior to spawning, showed that reproductive investment in males was much lower than in females.

The differences in size-specific fecundity in relation to size/age at maturity, and in reproductive and somatic investment were analysed for female plaice caught in four regions of the Irish Sea (Cumbrian coast, Liverpool Bay, Cardigan Bay and western Irish Sea), each of which contains a spawning focus. Both the reproductive investment (gonad weight as a function of body size) and fecundity–size relationship of plaice in the western Irish Sea were significantly different from those in the other regions. Rates of annual somatic growth appeared to fall into three distinct groups (Cumbrian coast; Liverpool Bay and Cardigan Bay; western Irish Sea) and, in all cases, the rate of somatic growth fell rapidly after maturity. The data suggest that the highest surplus production (as spawned eggs) occurs in the sub-populations of plaice on the Cumbrian coast and in Liverpool Bay, and is linked to reduced intra-specific competition for food.

Oocyte development was aborted and full maturity did not occur in virgin female sea bass Dicentrarchus labrax unless they remained in water above 10° C during the main period of gonad development. There was no difference in the condition factors of females with high or low IG values in March, which indicates that retarded gonadal development was not necessarily due to poor body condition or nutritional state. It is suggested that the extended duration of the adolescent phase of female bass around southern Britain is a response to their environment, rather than an intrinsic aspect of the species'biology.

Reproductively mature female plaice were implanted with or without 50 μg of gonadotrophin‐releasing hormone analogue (GnRHa), suspended in either coconut oil or methacrylate resin. The weight of the GnRHa‐treated fish increased significantly (due to hydration of the oocytes) and reached a peak between 10 and 14 days. The fish produced several batches of eggs, which were consistently bigger than those produced by control fish. Plasma concentrations of free 17β‐oestradiol and glucuronidated testosterone rose briefly (4 days) in response to the GnRHa, but then fell continuously till the end of the experiment (20 days). Plasma concentrations of sulphated 5β‐pregnane‐3α,17,20β‐triol and 5β‐pregnane‐3β,17,20β‐triol (which are putative metabolites of 17,20β‐dihydroxy‐4‐pregnen‐3‐one, the oocyte maturation‐inducing steroid) increased significantly at 4 days and reached a peak between 12 and 16 days. Concentrations were still very elevated on day 20. Plasma concentrations of sulphated 3α,17,21‐trihydroxy‐5β‐pregnan‐20‐one showed a slight increase on day 4 but did not change thereafter. There was a highly significant difference in the amounts of GnRHa released into the bloodstream by the two methods of administration on day 4. However, this was not matched by significant differences in the concentrations of any of the steroids.

Blood plasma concentrations of free 17β‐oestradiol, free testosterone and glucuronidated testosterone were strongly positively related to the percentage of vitellogenic oocytes remaining in the ovaries of plaice Pleuronectes platessa caught at sea–being at their highest in pre‐spawning (stage IV) females (i.e. those in which the oocytes were close to fully grown, but had not yet entered the stage of final maturation). In contrast, the concentrations of free and sulphated 17,20β‐P, 3αaL,17, 20β‐P‐5β, and 3α,17,21‐P‐5β were at their lowest in stage IV females. Free 17,20β‐P (the putative maturation‐inducing steroid) became only slightly elevated (less than twofold) during spawning (i.e. in stage V and VI females with hydrated and/or ovulated eggs). Sulphated 17,20β‐P and 3α,17,21‐P‐5β became slightly more elevated (three‐ to fourfold). However, sulphated 3α, 17,20β‐P‐5β concentrations increased 30‐fold and were at their highest in fish in which only 40% of vitellogenic oocytes remained in the ovaries. Sulphated 17,20β‐P, 3α, 17, 20β‐P‐5β and 3α,17,21‐P‐5β concentrations were significantly positively related to hyaline oocyte batch size; and sulphated 17,20β‐P and sulphated 3α, 17,20β‐P‐5β were significantly negatively related to the degree of hydration of the hyaline oocytes. None of the steroid concentrations, however, was related to the time of capture. More ovulated females were found in the afternoon than at any other time of the day.

Sexually mature Arcto-Norwegian female cod, Gadus morhua, were sampled off northern Norway either during spawning migration (Vesterålen) or at spawning sites (Lofoten) from 1986 to 1996. This period comprised a dramatic, nearly cyclical change in the Barents Sea ecosystem. The stock of the main food item, viz. the Barents Sea capelin Mallotus villosus villosus, changed from a low (1986), to a high (1991) and again to a low (1994) level of abundance while the climate changed from a cold (≤1989) to a warm regime. The relative annual potential fecundity (i.e. number of vitellogenic oocytes per g prespawning fish) increased by approximately 40% from 1987 to 1991. However, information from a back-calculation technique calibrated in the laboratory using spawning fish indicated that this change might have been as high as 80 to 90%. Ovaries were analysed by the gravimetric, the automated particle counting and the stereometric method (modified to use with ovaries too large to section whole). All three methods gave similar fecundity estimates. The latter method was applied to quantify atresia of developing oocytes in the good-condition year of 1991. Atresia was rare, occurring in only 30% of the ovaries and where it was present in only 1 to 4% of the vitellogenic oocytes. Spawning females sampled from 1991 to 1996 gradually produced fewer eggs and demonstrated clear interannual variations in vitellogenic oocyte mean size and distribution thought to reflect a delicate reproductive tactic to minimise negative nutritional effects on egg size and egg quality. Estimates of annual potential fecundity for the duration of the study were significantly positively correlated with environmental temperature and the availability of capelin during vitellogenesis.

Maximum oocyte size was used to assess seasonal ovarian development in sole. Fish age, especially the adolescent period, appeared to affect the start of vitellogenin-dependent oocyte development in the annual reproductive cycle and the subsequent oocyte growth rate. The majority of oocyte growth occurred between September and March. Several other aspects of ovarian development were also age-dependent, including the increase in ovary condition factor (ovary weight/fish length(3)) and the size of oocytes commencing nuclear migration. Evidence is presented that in the recruiting year class of sole abortive maturation occurs where oocytes develop yolk but spawning does not take place. The implications of this study on the estimation of female spawning stock biomass are discussed.