Population Reproductive Potential Research Articles

Effect of starvation on physiological and survival traits of Mimachlamys varia (Linneaus, 1758)

In order to better understand Mimachlamys varia (Linneaus, 1758) response to nutritional stress, a controlled-condition experiment was conducted. Two scallop batches (i.e. juveniles and adults) were completely food-deprived for 3 months. Changes in mass and energy content of tissues (adductor muscle, digestive gland, rest of the soft tissues), as well as stable carbon and nitrogen isotope ratios of the adductor muscle were monitored weekly. Both batches exhibited a 3-phase response to starvation. For adults, phase 1, was characterized by a fast loss in mass, an δ15N-enrichment, a stable calorific power, and a low mortality, corresponding to a transitional stage associated with protein-storage consumption. Phase 2 (day 28–42) exhibited a stabilization of mass and δ15N values, coinciding with a digestive gland calorific power drop and an acceleration in mortality. This corresponds to a “protein sparing” stage where highly energetic fuel such as lipids, which are stored especially in the digestive gland in pectinids, are consumed in priority. Juveniles exhibited a distinct response characterized by a significant mass loss and an increase in calorific power during the first phase (day 0–28). This body weight decrease may involve the remobilization of low-caloric biochemical compounds (e.g. proteins) using the structure as internal fuel, thus limiting somatic maintenance costs. During the second month, body mass and calorific power stabilized, indicating a “protein sparing” stage. In a third phase for both age classes, mass decreased again together with a sharp increase in mortality: essential structure was ultimately remobilized. The survival rate of juveniles was higher than that of adults during the first two months of the experiment (97% and 64%, respectively). Although the time required for starvation to deplete of half the cohort is higher for juveniles than for mature individuals, increase in mortality rate after reaching the “Point of No Return”, (e.g. the beginning of the last phase) was higher in juveniles than in adults. These results highlight the potential impacts of starvation at the population level, beyond the direct impact on individual survival. For instance, individuals may not effectively contribute to reproduction during the spawning period if exposed to a prolonged winter starvation episode. In particular, since highly energetic compounds stored in the digestive gland are often used as fuel to initiate gametogenesis in pectinids. Similarly, decrease in somatic weight in juveniles may delay their sexual maturity and hence their ability to contribute to population reproductive potential.

Intensive sea urchin harvest rescales Paracentrotus lividus population structure and threatens self-sustenance.

The harvest of the edible sea urchin Paracentrotus lividus is intensively practiced in some regions of the Western Mediterranean Sea. The removal of the largest individuals can determine an overall reduction in population size and a size class truncation that can lead to a drastic drop the self-sustenance. The aim of this study is to evaluate the variability of the population reproductive potential across 5 years in one of the main harvest hotspots of Sardinia (Western Mediterranean Sea). The breeding stock consists of commercial and under-commercial size individuals which were sampled on a monthly basis to estimate their GonadoSomatic Index (GSI) and the Individual Gamete Output (IGO). In addition, the reproductive potential of the population-Total Gamete Output (TGO)-was calculated across the 5-year period in relation with the variation of the density of the breeding stock. During the last year, the reproductive potential was also estimated in a well-conserved population of a nearby Marine Protected Area. No significant variability in GSI and IGO was found over the 5 years nor when compared with the ones of protected population in the last year. However, the intensive harvest drastically rescaled the population body-size: although density of the commercial size class remained low, density of the under-commercial size-class halved from the beginning to the end of the study. Accordingly, the proportional decrease of their gamete output contribution led to a 40% loss of the reproductive potential of the whole population in the 5-year period. Interestingly, despite the loss of reproductive potential due to the decrease of the breeding stock density, the average values of IGO slightly increased across the years leading to the highest Annual Gamete Output (AGO) during the fourth year of sampling. This positive pattern could suggest a mechanism of reproductive investments of the survivors in terms of gonad production rate or increase in spawning intensity. This work provides evidence of the direct effect of size-selective harvesting on the rapid loss of population self-sustenance. Furthermore, it lays new prospective for future research of the indirect effects of the rescaling population body-size in functional traits of the sea urchin P. lividus and that could become important for both, sustainable exploitation and ecosystem conservation management.

A Bayesian Multilevel Ordinal Regression Model for Fish Maturity Data: Difference in Maturity Ogives of Skipjack Tuna (Katsuwonus pelamis) Between Schools in the Western and Central Pacific Ocean

The maturity ogive is vital to defining the fraction of a population capable of reproduction. In this study, we proposed a novel approach, a Bayesian multilevel ordinal regression (i.e., Bayesian continuation ratio model), to model the maturity ogive. The model assumes that the observed maturity stage originates from the categorization of latent continuous variables. We demonstrated this approach by testing whether there are differences in the maturity ogive of skipjack tuna (Katsuonus pelamis) in the western and central Pacific Ocean between two school types, i.e., free-swimming and floating-object-associated schools. The model results show that K. pelamis, given the same fork length, are more likely to have a higher maturity stage in a free-swimming school than those associated with floating objects. The gonadosomatic index revealed the same conclusion. Our results indicate that fish aggregation devices (FADs) could negatively affect the maturity of K. pelamis and consequently reduce the population reproductive potential. This study provides (1) an alternative approach to analyze fisheries ordinal data; (2) important quantitative evidence to evaluate the existing ecological hypotheses; and (3) implications for tuna fisheries management.

Consequences of trait-selective fisheries on population reproductive potential: An experimental approach

The vulnerability of fish to fishing depends on a range of life-history (e.g. growth; reproduction), behavioural (e.g. boldness) and physiological (e.g. metabolic rates) traits which are usually correlated with reproductive success. Therefore, between-fish differences in catchability may be indirectly affecting the reproductive potential at the population level by removing certain phenotypes. The present study aims to test this hypothesis by assessing the relationship between vulnerability-to-angling and reproductive potential. We first scored the vulnerability-to-angling of a set of Serranus scriba adult individuals (n = 78), a species targeted by recreational fisheries, using intensive multi-event tests. The fish were then divided into three categories: high, medium and low vulnerability. Fish differing only in their vulnerability degree (i.e. the same average size and culture conditions) were kept in tanks (n = 6; two tanks per category). We monitored the tanks throughout a full spawning season to assess differences in seasonal distribution of spawning, total egg production, egg viability and egg quality. Fish with different vulnerability showed no significant differences in egg production (in terms of spawning seasonality and total eggs released) nor on egg viability compared to vulnerable fish. However, low vulnerability fish produced eggs with bigger egg yolk compared to high vulnerability fish but only toward the end of the fishing season. We interpret this difference as part of a portfolio effect in terms of behaviour and reproductive success, i.e., in unexploited populations, low vulnerability fish may contribute little to year class strength in typical years but act as a buffer, if a disturbance affects reproductive success early in the season. Moreover, although our work is based on a captivity experiment, the results are consistent with the hypothesis that harvested populations may compensate for the harvested biomass by investing more energy per fish in reproduction.

The reproductive value of large females: consequences of shifts in demographic structure for population reproductive potential in Chinook salmon

While conservation and fisheries management are often concerned with changes in population abundance and distribution, shifts in population age–size structure are commonly observed in response to human and environmental stressors. Chinook salmon (Oncorhynchus tshawytscha) have experienced widespread declines in mean age and size throughout their North American range. We investigated the consequences of declines in body size for spawner reproductive potential in terms of total egg mass per female. Our case study is the Yukon River where Chinook salmon have supported subsistence, commercial, and recreational fisheries. Using historical observations on individual body size from throughout the Yukon River and the relationship between female size and total egg mass from the Canadian portion, we estimate a decline in average female reproductive potential of 24%–35% since the 1970s. Because spawner abundances and the population sex ratio have not shown clear trends over time, our results suggest a reduced total population reproductive potential. Changes in spawner quality should be considered when developing management reference points, and conservation of population demographic structure may be necessary to sustain productive Chinook salmon systems.

Многолетняя динамика репродуктивных показателей европейского анчоуса Engraulis encrasicolus (L., 1758) и их связь с температурными условиями

Европейский анчоус, или хамса (Engraulis encrasicolus L.), — один из наиболее массовых видов рыб в Азово-Черноморском бассейне. Является основным промысловым объектом: доля его вылова составляет около 80 %. Оценка состояния популяции хамсы и её динамики в условиях климатического потепления — одна из актуальных задач современных исследований. Данная статья посвящена изучению многолетней динамики ряда популяционных репродуктивных показателей хамсы и их связи с температурой воды с целью возможного использования для прогноза состояния популяции при изменении условий среды. В основе работы – полученные в 2000–2015 гг. результаты собственных исследований, выполненных в прибрежной акватории Чёрного моря, прилегающей к юго-западному побережью Крыма (район Севастополя — Балаклавы). Для сбора икры хамсы использовали планктонную сеть Богорова — Расса (БР-80/113) в режиме вертикального лова в слое 0–10 м. Взрослые экземпляры получены из ставных неводов. Проанализированы 702 ихтиопланктонные пробы и 941 экз. нерестовых самок (на стадиях зрелости V, VI–IV и VI–V). Среди популяционных репродуктивных показателей были изучены календарные сроки и продолжительность нерестового периода, интенсивность и эффективность нереста, внутривидовой состав родительского стада. Календарные сроки нереста хамсы в 2000–2014 гг. у юго-западного побережья Крыма варьировали от второй декады мая (ранний нерест) до третьей декады июня (поздний нерест); сроки окончания — от третьей декады августа до третьей декады сентября. Общая продолжительность репродуктивного периода составляла 8–14 недель, средняя — 11 недель. В многолетнем плане установлен сдвиг начала нереста на более ранние календарные сроки. Максимальные (пиковые) среднемесячные значения интенсивности нереста в 2012–2013 гг. варьировали от 33.6 до 78.7 % (среднее 51.2 %). Пик популяционного нереста приходится на июль и август; его повторяемость в июле составила 25.0 %, в августе — 66.7 %. В многолетнем плане установлен сдвиг пика нереста на более ранние сроки: в 2000–2005 гг. повторяемость пика нереста в августе составляла 100 %, в 2007–2013 гг. — только 50 %. Абсолютная численность икры (эффективность нереста) в 2000–2014 гг. изменялась от 1.6 до 29.9 экз.·м-2, составив в среднем 10.3 экз.·м-2. Условно выделены низко-, средне- и высокоурожайные годы с численностью икры менее 10, 10–20 и более 20 экз.·м-2 соответственно и со средней эффективностью нереста 5.1, 14.5 и 25.0 экз.·м-2. Эффективность нереста высокоурожайных лет в 1.7 и 4.9 раза превышает таковую средне- и низкоурожайных лет. Установлен многолетний положительный тренд эффективности нереста. В 2000–2011 гг. произошла перестройка внутривидовой структуры хамсы, которая сопровождалась перераспределением численного соотношения в составе нерестовой части популяции представителей азовского и черноморского подвидов в пользу последнего. До 2004 г. относительная численность черноморской хамсы составляла 33.3 %, после 2004 г. — 76.7 %. Одновременно произошло заметное увеличение её вылова. Коэффициент корреляции между относительной численностью черноморской хамсы и выловом составил 0.92. Календарные сроки начала нереста, его интенсивность и эффективность тесно связаны с температурой воды. Нижней температурной границей массового нереста является 17.5 °С. Область «максимально благоприятной нерестовой температуры» находится выше 23 °С. В её пределах реализуется около 2/3 репродуктивного потенциала популяции.

Goosefish Lophius americanus fecundity and spawning frequency, with implications for population reproductive potential.

To improve knowledge of goosefish Lophius americanus' reproductive biology, females were collected during 2009-2012 from the Mid-Atlantic Bight shelf region of the U.S. east coast. Batch fecundity increased with total length (LT ), from 229 100 to 2 243 300 mature oocytes per female (LT range: 55·5-112 cm; n = 54). This estimate of fecundity at LT is lower than one derived from a sample collected during 1982-1985. Examination of whole oocyte diameters in different months indicated that L. americanus is a serial spawner, releasing more than one egg veil per spawning season, as suspected or observed for other Lophius species. Seasonality of spawning was evident from whole oocytes and gonad histology, and from larval fish surveys spanning the U.S. north-east shelf, and confirmed a protracted (c. 6 months) spawning period. Peak spawning activity progressed northward from spring to autumn. The population-level implications of these results were explored by estimating population reproductive potential (PRP ), which considered the value of both current and future per capita reproduction using decade-specific age structure and fecundity at length. PRP is now more than 50% lower compared with the historical period (1982-1985), a result of the lower proportions of large females and reduced fecundity across all sizes. Mechanisms that could explain this loss of stock productivity are fishing-induced size-age truncation or regime shifts in egg production caused by changes in energy density of common forage species.

Fecundidad de la medusa <i>Stomolophus meleagris</i> (Rhizostomeae: Stomolophidae) en el Golfo de California

The cannonball jellyfish Stomolophus meleagris is regarded as a fishery resource with high commercial value, but with scarce biological information. With the aim to generate preliminary information on reproductive aspects, the present study analyzes its fecundity, based on the estimated number of vitellogenic oocytes in the gonad; in addition, we evaluated its relationship with the jellyfish body length, diameter and wet weight; and we established the relationship of the gonadosomatic index (IGS) with the jellyfish diameter and length. For this, a total of 30 specimens were collected, measured and weighed in a monthly basis, in Las Guasimas lagoon from January to May 2006. In order to estimate the gonad volume, 60 mature females were analyzed and their gonads were extracted and stained with hematoxylin-eosin. Gonad images from histological preparations were analyzed and the number of vitellogenic and previtellogenic oocytes obtained. We used catch data from The Guasimas for population recruitment analysis, that was estimated with FISAT II. Among results, we found that fecundity increased with jellyfish length, diameter and total wet weight. The lower fecundity rates occured on February and the highest during May (11 873 071 and 37 528 197 millions of vitellogenic oocytes per gonad, respectively). The IGS increased with length and diameter of the specimens, at its maximum value of 3.7% was observed in May. A continuous production of previtellogenic oocytes were observed during the full collection period. The maximum recruitment was observed in July and November (37.50% and 28.01% respectively). The high fecundity observed in S meleagris, suggests a high population reproductive potential that may support the possibility of an important fishery consolidation on this jellyfish.

A Standardized Method and Analytical Approach for Predicting Female Reproductive Stage in Teleosts by Using Ovary Color and Female Characteristics

AbstractDetermining and understanding patterns in the reproductive status of fishes are essential for assessing individual and population reproductive potential. The least biased method for determining reproductive stage is through the use of ovarian histology; however, this method can be time consuming and expensive. To overcome these restrictions, we developed models to predict the reproductive stage of female striped bass Morone saxatilis by using ovary color and other female characteristics. We provide a standardized and calibrated method for quantifying ovary color, and we outline an analytical approach that utilizes binary and ordinal logistic regression. Our results indicated that binary models had high accuracy (<6% error) in distinguishing between the regressing phase (i.e., recently spawned) and the nonregressing phase (i.e., all other reproductive phases); we found even better accuracy (4% error) for ordinal models in discriminating among four nonregressing reproductive phases. All of the best‐fitting models required the inclusion of an ovary color variable and ovary energy density; however, good predictive accuracy was also obtained when we replaced ovary energy density with ovary percent water to produce models requiring minimal laboratory processing time and cost. Although tested on striped bass, our method could be used to develop similar models for other species.

Integrating fish size, condition, and population demography in the estimation of Atlantic sardine annual fecundity

Spatial and temporal variability in reproductive parameters was studied in combination with population demography to estimate sardine (Sardina pilchardus) population annual fecundity, as an indicator of its reproductive potential. Samples collected from commercial landings were used to determine the monthly proportion of active females and the duration of the spawning season, while data obtained during the 2002, 2005, and 2008 Daily Egg Production Method surveys were used to analyze the relationship between reproductive parameters (batch fecundity, spawning fraction) and individual biological characteristics (length, gutted weight, condition) for the west and south coasts of Portugal by generalized linear models. Population abundances and biological characteristics obtained per length class during acoustic surveys were subsequently used to integrate individual egg production to population demography. At the individual level, results indicated that the duration of the reproductive season was size-dependent, being shorter for smaller sardines. Batch fecundity was positively related to body size and differed significantly between the years and areas studied, with higher values on the west coast and an increasing trend towards the 2007/2008 spawning season. Mean values of spawning fraction varied significantly with space and time but the poor explanatory relationship obtained by the modelling of the spawning fraction against the individual biological characteristics did not allow the clarification of the factors affecting sardine spawning activity. At the population level, the differences obtained in egg production were found to be related to changes in population abundances as well as to the demographic structure, as larger females contributed proportionally more to the population annual fecundity. These results are in accordance with numerous recent studies indicating that the population reproductive potential may not always be proportional to the spawning biomass.

Pattern and causes of a temperature-dependent gradient of size at terminal moult in snow crab (Chionoecetes opilio) along West Greenland

Geographic variation in size of male and female snow crabs (Chionoecetes opilio) was investigated along the west coast of Greenland to test the hypothesis that size at terminal molt (=adulthood) is temperature dependent. A total of 81,490 snow crabs were collected in small-mesh traps in Disko Bay (68–69°N) and six sites near Sisimiut (66–67°N) in May and June from 2000 to 2005. Average bottom temperature over the study period ranged from −0.8 to 3.2°C across the sampling sites. Mean carapace width was positively correlated with temperature in both sexes, a pattern often described as a converse Jame’s cline. We infer that temperature per se is the causative factor and discount season length, food availability or density as ultimate causes of the cline. Temperature effects on body size of crabs apparently result from a change in the number of instars before terminal molt. This interpretation is supported by size frequency analysis showing that in general crabs were larger at instar in a colder than in a warmer site. We briefly discuss the implications of our findings for population reproductive potential and the effectiveness of a fixed legal size limit in protecting some adult males from exploitation at different temperature regimes.

Sex‐Specific Natural Mortality of Arrowtooth Flounder in Alaska: Implications of a Skewed Sex Ratio on Exploitation and Management

AbstractConservative fisheries management in Alaskan waters typically limits harvest to maintain a target biomass of female spawners (40% of the female biomass observed in the absence of fishing). This harvest calculation does not consider the male population level for a species. If natural mortality rates (M) differ between sexes and if females outlive males, then a reduction in males relative to females could diminish the reproductive potential of the stock. This problem was examined in the present study by estimating separate sex‐specific M‐values for the arrowtooth flounder Atheresthes stomias in Alaska, where females are almost always found at higher proportions than males during trawl surveys. Age data indicated that females outlive males, and estimates of M ranged from 0.10 to 0.33 for females and from 0.16 to 0.51 for males. Based on four estimation methods, male M was consistently higher than female M. Simulated harvest scenarios in which male M and selectivity were varied indicated that increasing male M and lowering the age of males at first capture skewed the sex ratio even more toward females; in the most extreme harvest scenario, the percentage of females in the population was as high as 89% (Gulf of Alaska) and 96% (Bering Sea). Because reproductive success under a skewed sex ratio is probably linked to reproductive behavior, we developed a simple conceptual model to consider three reproductive strategies that differed in the number of females (1–3) that could be paired with a single male. Although a population's reproductive potential was much more resilient to changes in sex ratio for 1:2 or 1:3 (male : female) pairings (reduced by only 15% when the population was 70% female), the fishing mortality rate (F) that corresponded to a reduction in egg fertilization was much higher than the target F (i.e., that maintained biomass at 40% of the unfished biomass) and exceeded overfishing levels when the population was over 80% female.

Reproductive Success of Multiparous Female Tanner Crab (Chionoecetes bairdi) Fertilizing Eggs With or Without Recent Access to Males

Sperm storage in female crabs is assumed to provide resiliency in population reproductive potential for exploited crab stocks with male-only harvest restrictions. However, contrasting results have been reported for the reproductive success of female Tanner crab (Chionoecetes bairdi) relying on stored sperm for fertilization. Ovigerous multiparous female C. bairdi were collected and held in the laboratory in the absence of males from hatching to extrusion or collected after hatching and extrusion of a subsequent clutch in situ in Glacier Bay, Alaska. Females were classified into two groups based on extrusion site, LE (laboratory) or FE (field). Mating of multiparous females was important for maintenance of reproductive potential as sperm reserves were likely insufficient for full fertilization of an additional egg clutch from stored sperm in 44% of LE females. High incidence of fresh grasping marks for FE (97%) versus LE (4%) females and significantly higher sperm reserves indicated that most FE females were likely to have recently mated. Mean postextrusion spermathecal load and sperm cell counts increased significantly from 0.21 to 0.44 g and 2.00 × 107 to 9.35 × 107, respectively, between LE and FE females (p 99%), with similar fecundity. Characterization of spatiotemporal trends in mating status, sperm reserves and egg production in C. bairdi may be valuable for understanding natural variability in reproduction and possible fishery effects on population reproductive potential.

The impact of intrapopulation variability in reproductive traits on population reproductive potential of Grand Bank American plaice ( Hippoglossoides platessoides) and yellowtail flounder ( Limanda ferruginea)

In geographically extensive fish populations the potential exists for reproductive traits to vary over the population’s range but, the impact that such intrapopulation variability has on overall population reproductive potential has not been formally assessed. Here intrapopulation spatial variability in size at maturity and fecundity are demonstrated for Grand Bank American plaice ( Hippoglossoides platessoides) and yellowtail flounder ( Limanda ferruginea). Recognition of intrapopulation variability in these reproductive traits coupled with spatial variability in abundance resulted in an increase in estimated population total egg production (TEP) by as much as 10 14 eggs for American plaice and 10 15 eggs for yellowtail flounder as compared to assessment of TEP for the population as a whole. Results highlight the need to explore variability in life history traits not only between but, also within populations and emphasizes the need for sufficient spatial coverage during sampling in order to assess the reproductive potential of fish populations.

Changes in Embryonic Development and Hatching inChionoecetes opilio(Snow Crab) With Variation in Incubation Temperature

Water temperature affects the distribution, movement, and reproductive potential of female snow crab, Chionoecetes opilio. Ovigerous females of C. opilio from the eastern Bering Sea were held at five temperatures (-1, 0, 1, 3, and 6 degrees C) in the laboratory while their embryos developed from gastrula to hatching. The duration of incubation increased by 105 d (30%) with decreasing temperature; however, a switch to a 2-year duration of embryo incubation was not observed. For females held at 6, 3, and 1 degrees C, their embryos underwent a short period of diapause late in development; no diapause was observed for embryos of females held at 0 or -1 degrees C. Successful extrusion of a subsequent clutch and hatch timing comparable with that observed in the eastern Bering Sea indicated that temperatures of 0 to 3 degrees C may be optimal for multiparous female reproduction. We demonstrated that a switch from 1-year to 2-year reproduction cannot be triggered by changing the thermal regime after several months of embryonic development. The timing of female movement from colder to warmer waters may be important for maintaining population reproductive potential during the recent phase of warming and contraction of cold-water biomes in the Bering Sea.

Conservation and management of exploited shark populations based on reproductive value

Several life history traits of sharks result in juveniles being particularly vulnerable to exploitation. However, population level impacts of harvests on juvenile sharks have not been well quantified. This paper examines a range of harvest strategies, including those targeting juveniles. Reproductive value and yield per recruit are used to compare the harvests, which are represented by Leslie matrix models with a harvest matrix. Two species are used as examples: the short-lived Rhizoprionodon taylori and the long-lived Squalus acanthias. Harvests that maintain a stationary population size cause reproductive values to change in opposing ways, but they remove equal fractions of the population's reproductive potential. A new theorem gives population growth as a function of the fraction of reproductive potential removed by a harvest, a relationship useful for comparing harvests on juveniles and adults. Stochastic projections indicate that the risk of depletion is associated with the fraction of reproductive potential removed annually, a measure which encompasses the information in both the selectivity and the rate of fishing mortality. These results indicate the value of focusing conservation efforts on preserving reproductive potential.

The relationship between fish condition and the probability of being mature in American plaice (Hippoglossoides platessoides)

Abstract Fish condition can be related to a population's reproductive potential in a variety of ways. The relationship between condition and the probability of being mature (adult) was examined in three populations of American plaice (Hippoglossoides platessoides). The effect of condition was tested after first removing the known effects of age and length. Neither relative liver condition nor relative body condition significantly affected the probability of male American plaice being mature, but there was a significant positive effect of both condition indices on the probability of being a mature female. However, the additional variation explained by female condition was small when compared with the combined effect of age and length. Condition is related to the age and size at which fish attain adulthood. Variation in fish condition will have a direct impact on the spawning stock biomass of a population through differences in the maturation schedule of cohorts with differing condition.

Population reproductive potential: Evaluation of long-term stock productivity

: In order to avoid recruitment overfishing, fish stocks must have sufficient reproductive ability. The spawning stock biomass (SSB), which ignores the value of immature fish, is widely used as an index of stock sustainability. From the perspective of sustainability, immediate reproduction, as well as future spawning, must be considered. We developed an index of long-term stock productivity, called the population reproductive potential (PRP). PRP is defined as the expected total reproductive value of the standing stock. We used PRP to assess the western Atlantic bluefin tuna (WBT) stock. The trends in SSB, numbers (N), biomass and PRP of WBT are inconsistent when compared to each other, due to fluctuation in age composition. We evaluated the long-term productivity of WBT by computer simulation and compared the result with trends in the abundance indices. The result of the computer simulation was highly consistent with the trend in the PRP. Short-term trends in SSB and N often do not reflect long-term stock trends, because they are highly sensitive to age-composition dynamics. The PRP is useful for evaluating stock trends, especially when the age composition is unstable.

Efficiency and ghost fishing of snow crab ( Chionoecetes opilio) traps in the Gulf of St. Lawrence

Experiments were conducted in the southwestern Gulf of St. Lawrence from 1988 to 1993 to: (1) evaluate the catch performance of three types of conventional commercial snow crab traps; (2) evaluate the catch rates of soft-shelled crabs in normal and modified conical traps; and (3) investigate ghost fishing by lost traps. Our results showed that pyramidal and conical traps captured larger-sized males than conventional rectangular traps. Pyramidal traps also obtained significantly higher catch rates of commercial male crabs than conventional rectangular traps. The recent increased use of conical and pyramidal traps may result in a considerable reduction of the most reproductively important component of the male population. Close monitoring of the population reproductive potential is necessary to ensure a healthy population in the long-term. Our experimental conical traps, modified by attaching a 18 or 24 cm plastic panel around the top, caught significantly less soft-shelled crabs while maintaining the same catch rate of commercial crabs as compared to the conventional conical trap. This modified trap will play an important role in avoiding unnecessary mortality of crabs and ensuring the long-term conservation of the stock. Results from the ghost fishing experiment suggested that baited snow crab traps set in summer captured snow crab to their saturation level. The number of crabs then slowly decreased due to cannibalism and predation over winter. The mortality rate of snow crabs trapped inside conical traps was estimated at 94.6% per year. The catches increased again to their saturation level before the following spring, due to the self-baiting effect, which re-initiated a ghost fishing cycle.

Impact of the v-notch technical conservation measure on reproductive potential in a lobster (Homarus gammarus L.) fishery in Ireland

In total, 7619 v-notched lobsters were released in the Wexford lobster fishery in Ireland between 1994 and 1999 in order to increase egg production. These lobsters were legally protected from fishing. Annual reproductive potential (RP) from these lobsters, based on numbers and size at release, was 2.6×107 eggs. Average ( 95% confidence limit) standing stock, between 1994 and 1999, from 31 Petersen population estimates was 29427 3871 female lobsters. Population RP was 6.9×107 eggs per year. After discounting numbers for natural mortality and incorporating increases in size of surviving lobsters, RP of v-notched lobsters in 1999 was 4.13×107 eggs per year. Catch rates of v-notched lobsters were correlated with numbers available for re-capture between 1994 and 1998. In 1999 catch rates were lower than expected, possibly because of repair and disappearance of the mark during moulting. Catch rates of undersized lobsters were 37% higher in 1998–99 than in 1994–97. In highly exploited small-scale lobster fisheries, substantial increases in RP can be achieved by v-notching. Demonstrating a dependency between local increases in RP and local recruitment is problematic and requires more information on the early life history and the recruitment process.