Stock Recruitment Research Articles

Occurrence and Strength of Instantaneous and Intracohort Density-Dependence in Northeast Atlantic Fish Stocks.

Biological reference points (BRPs) used in fisheries management do not include density-dependent (DD) growth, with DD processes only considered in the stock recruitment relationship. Not accounting for DD on somatic growth has led to criticism that such BRPs underestimate the compensatory effects of DD at low stock size, and therefore risk foregone catch opportunities. Here, we analyse 81 stocks from the Northeast Atlantic for evidence of DD growth, defined as the process in which stock size affects somatic weight. We evaluate the following questions: (1) How many stocks have experienced instantaneous DD growth and do stocks of the same species display similar trends? (2) Is there a common instantaneous DD growth relationship shared by all stocks? (3) For stocks exhibiting significant instantaneous DD growth, can we quantify the strength of the relationship? (4) Is DD growth operating as an intra-cohort process as opposed to an instantaneous effect? Results reveal that only the weight of recruits exhibits a common instantaneous DD growth while the other responses analysed show a positive, noncompensatory effect, suggesting that other processes are at work. All responses examined showed significant temporal autocorrelation, which, when not accounted for, suggest apparent instantaneous DD growth in several stocks. Comparison of instantaneous against intracohort DD growth showed an increase in the number of stocks with significant DD growth, although, as for instantaneous DD growth, this declined greatly when temporal autocorrelation was accounted for. Our results counteract the a priori assumption that DD growth compensation is related only to stock biomass or density, suggesting that DD growth should be dealt case-by-case. Consequently, management practices that aim to fish down stock biomass with the anticipation of triggering DD growth will be associated with greater asymmetric risks than keeping biomass at levels where replacement yield does not rely on it.

Description of regression models for predicting the dynamics of pink salmon returns in the Kamchatka region based on climate-oceanological and population-genetic data

Several regression models for predicting returns of pink salmon in the Kamchatka region are presented. The data for 1990–2023 were analyzed. Among available climatic and oceanological indices, the most suitable for using as predictors for forecasting of pink salmon returns were the Pacific Decadal Oscillation (PDO) index, Western Pacific Cyclonic Index (WP), Arctic Oscillation (AO) index, and the sea surface temperature anomaly in the North Pacific. Multi-dimensional models of the «stock–recruitment» type were built on identified statistical patterns, which allowed to estimate potential abundance of the pink salmon returns to northeastern and western Kamchatka. Besides, methods for predicting the abundance of pink salmon returns on the data of fish counting in the sea are considered, using the materials of TINRO trawl surveys conducted in the Bering and Okhotsk Seas in the fall seasons of 2012–2023. To determine the abundance of pink salmon originated from West Kamchatka, genetic identification of regional composition of juveniles in mixed trawl catches was used. All tested methods have a high level of determination, but simpler regressive models are more prospective for practical forecasting of general trend in dynamics of pink salmon stocks in the Kamchatka region due to very weak generalization ability of more complicated models.

Simulated reproductive allocation to fisheries-induced evolution among small yellow croaker populations in the Yellow and Bohai Seas

Fishery-induced evolution (FIE) affects the body size and reproduction of offspring, resulting in feedback regulation of fish survival and stock recruitment. Small yellow croaker (Larimichthys polyactis) is a species of high ecological and commercial importance in the Yellow and Bohai Seas, with large fluctuations in abundance linked to overfishing and environmental changes in recent decades. To better understand the maturation size and age, threshold reproduction cost, and energy trade-offs of small yellow croaker under environmental changes (e.g. conditions) and human impacts (e.g. fishing) in the Yellow and Bohai Seas, we utilised a two-pool uncertain growth model containing the growth threshold of reproductive energy and reproductive energy cost. The results showed that for growth, maturation age, and reproduction under the baseline scenario based on field survey data from the 1950 s, when resources for small yellow croaker were underexploited, the average age was 5–6 years old and the average body length was approximately 220 mm. As the model parameters for fishing pressure and condition mortality increased, small yellow croaker tended to mature earlier, had a smaller body size, and allocated more energy to reproduction, resulting in higher population fecundity but shorter lifespan. Higher reproductive costs led to a greater proportion of reproductive energy being allocated during the spawning period, resulting in an older age of sexual maturity, shorter lifespan for spawning populations, and poor physical conditions, resulting in higher post-reproductive mortality and a limited capacity for stock recruitment. The key life-history traits of small yellow croaker, including maturity age, mean age of the population, age-specific body weight, and spawn production at different age stages, can change under different conditions. The model represents the main life history strategies and energy allocation characteristics of small yellow croaker. The simulated age and body size were within the observed range and the predicted fecundity aligned with the observed values. This study provides a prediction tool for protecting the spawning and recruitment of small yellow croaker and establishes a biological reference for sustainable management of small yellow croaker populations in the Yellow and Bohai Seas in the future.

Enhancing the utility of known-biomass production models: a case study of the Bay of Biscay and Iberian Coast ecoregion

Our general purpose is to support the use of known-biomass production models (KBPMs), illustrating their usefulness by addressing the evolution of surplus production (SP) over time and the factors affecting it (e.g. environment). We also demonstrate the utility of KBPMs for multispecies management objectives or for estimating maximum sustainable yield reference points without a stock recruitment function, among other worthwhile applications. To do so, we present different uses of KBPMs, illustrating their application on demersal species in the International Council for the Exploration of the Sea (ICES) area, specifically for megrim, white anglerfish and European hake stocks. The proposed analytical approach involved fitting single-species and multispecies KBPMs, conducting retrospective analyses and assessing the effects of environmental variability on SP. The findings show that, in general, stock SP increased after a decline in biomass and SP, except for white anglerfish in the southern area. Megrim stocks are the least productive, while hake and northern anglerfish are the most productive. Retrospective analysis revealed SP shifts in northern hake stock for reasons other than biomass variability. Hence, the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation (AMO), two key climate variability modes in the North Atlantic, were tested for their links to SP, revealing a positive connection between SP and AMO, although further research is necessary. Beyond the specific results of our particular KBPM application, our main conclusion is that KBPMs can serve as a tool complementary to more complex assessment models for resolving unaddressed issues and crosschecking available assessment results.

The response of marine fish population productivity to juvenile habitat modifications depends upon life histories

AbstractCoastal and estuarine habitats are essential for both growth and survival at juvenile stages for a large proportion of marine fish but are exposed to a variety of threats. However, the quantitative impacts of anthropic‐mediated nursery degradation on exploited population dynamics and productivity are rarely addressed. We developed a comprehensive steepness‐based parameterization of the stock–recruitment relationship that directly depends upon juvenile habitat quality and surface area and used a generic age‐ and stage‐structured model to simulate exploited population dynamics under nursery degradation/restoration scenarios. Population responses to juvenile habitat modification were estimated for three species with contrasting life histories. Modifying the surface area of nursery habitat has a predictable direct scaling effect on population size and affects biomass and fishing yield, with identical responses observed for various life histories. Modification of habitat quality affects both the population size and the shape of the productivity curve, leading to more complex responses of biomass and fishing yields that depend upon combinations of life history traits. Populations with low steepness and hence lower resiliency to fishing are the most sensitive to the degradation of habitat quality. Our study emphasizes the critical impact of juvenile habitat modification on population dynamics and productivity and the need to integrate habitat consideration into the management of exploited marine species. Our modelling framework is generalizable and can be extended to handle many diverse life histories and case studies to quantify the consequences of coastal habitat degradation/restoration for the dynamics of exploited marine fish populations.

Predation on Baltic sea yolk-sac herring larvae (Clupea harengus) by the invasive ctenophore Mnemiopsis leidyi

Western Baltic spring spawning herring (Clupea harengus, L.) is a commercially important fish stock currently suffering a strong decline. Larval survival is essential for stock recruitment and can be substantially decreased by predation. The comb jelly Mnemiopsis leidyi A. Agassiz, 1865, is a lobate ctenophore which is invasive to the Baltic Sea and a known ichthyoplankton predator. However, predation on herring larvae in the Baltic Sea by M. leidyi has not been studied since its initial establishment in 2006. To address this knowledge gap, we conducted feeding experiments to investigate (1) the predation capability of M. leidyi on herring yolk-sac larvae, and (2) the influence of larval age, (3) predator size and (4) the presence of alternative prey on the clearance rate of M. leidyi on herring yolk-sac larvae. Our results showed that M. leidyi exhibited the ability to capture and digest herring larvae. The clearance rates of M. leidyi on herring larvae decreased with larval age and increased with predator size, while the presence of alternative prey had no effect on clearance rates. This finding suggests that M. leidyi can efficiently consume herring yolk-sac larvae under laboratory conditions. However, further investigations using mesocosm or field studies are necessary to evaluate the potential impact of M. leidyi on the mortality and recruitment of herring yolk-sac larvae under Baltic Sea field conditions.

Study of the coastal reef fishery pressure in a South Pacific Island (Bora-Bora, French Polynesia)

In the context of the general overexploitation of South Pacific reef fisheries, there is a global push to precisely define fishery parameters such as fishing effort, annual landings, and exploitation levels. In Bora-Bora, several surveys were performed to define the fishing grounds and the fishing effort of the reef fishery for the first time. Maps of the fishing grounds under different fishing pressures were created for the Bora-Bora reef fishery. The average fishing effort throughout the lagoon was 40 trips week−1 km−2. Annual landings were estimated following two different methods: school participatory surveys and landing based on the catch per unit effort. These estimations were used to determine that the maximum sustainable yield of the island's fisheries was 6.5 tons km−2 year−1. the reported annual yield in 2022 was 5.35 tons km−2 year−1, under the estimated maximum sustainable yield. The level of exploitation of the fisheries was also estimated based on the catch size composition of the ten most fished species. This revealed that fishing is responsible for more than 50% of fish mortality in six of the ten studied species, and that catch length could be increased to optimize both catches and biomass. Overall, the Bora-Bora reef fishery may be experiencing growth overfishing (i.e., the catching of fish that are too small to maximise biological sustainability and economical yield), which can endanger stock recruitment in the future if no actions are taken.

Bridging proxy reference points to sustainable yields to support data-limited fisheries

Many of the world fisheries, especially small-scale fisheries, are in data-limited situations, for which proxy Biological Reference Points (BRPs) derived from per-recruit analyses are widely used to inform fisheries management. However, due to limited information on stock recruitment, per-recruit analyses usually assume constant recruitment and the influences of this key assumption have not been fully understood. In this study, we combined per-recruit analyses and stock-recruit relationship to build equilibrium models for eight species with different life history traits, and used the management strategy evaluation (MSE) framework to assess the uncertainty of proxy BRPs in achieving fisheries sustainability. Our results showed substantial variations in the performances of proxy BRPs on maintaining yield and biomass, and even conservative BRPs, e.g. F0.1 caused overexploitation in half of the cases (4 out of 8). The divergence of the performance between proxy BRPs and FMSY could be substantial, depending on steepness of stock-recruit relationships (correlation ranged from –0.55 to 0.9 for yield, and 0.53 to 0.96 for biomass). Accordingly, we presented an empirical approach for remediation, using correction coefficient to adjust the traditional proxy BRPs. The empirical approach showed satisfactory performances in the validation processes with an independent dataset. This paper demonstrates that proxy BRPs may introduce risks in achieving fisheries sustainability, whereas the errors can be readily adjusted with empirical estimates of steepness for most data-limited fisheries.

Testing novel methods for short-term forecasting of European glass eel recruitment

Recruitment forecasting constitutes a major issue in population dynamics, especially in stock assessments. In many cases, high recruitment stochasticity thwarts the determination of a stock-recruitment (SR) relationship. When no SR relationship is available, recruitments can be projected from past trends using methods such as the geometric means of the past recruitments. This approach implicitly assumes stability in upcoming years and might not be appropriate in the case of regime shifts. Recruitment forecasting is particularly critical when predicting fishing opportunities for recruitment fisheries. The fishery of glass European eels (Anguilla anguilla) is an example of a recruitment fishery. The French glass eel fishery, which is currently guided by two models to forecast upcoming recruitments and derive fishing opportunities, has failed to achieve management targets. This study develops new models that forecast glass eel recruitments without assumptions about the SR relationship, and with flexible assumptions about future recruitment evolutions. Tests based on multiple criteria found the best performance in a random slope model, which provides flexibility to track recruitment dynamics while balancing the interests of fisheries, management and conservation. This model has wide potential for trend projections in natural resource management, economics and other fields.

Stochastic modelling and synthesis of dynamic fish recruitment productivity in the Celtic Seas ecoregion

Abstract The Celtic Seas ecoregion (CSE) is undergoing climatic and ecosystem changes, which can induce changes in fish productivity. Globally, the productivity of many stocks has shown evidence of change over decadal timescales. Varying factors might drive these dynamics in the CSE, but for many stocks, these mechanisms have not been fully understood to be included in management advice. We study dynamic productivity for 28 stocks in the Celtic Seas by tracking integrated stochastic signals in the relationship between stock size and recruitment using state-space modelling applying Peterman’s Productivity Method. Our research objectives were to (i) fit Ricker stock–recruitment models with time-varying parameters to all age- or length-based assessed stocks in the CSE, (ii) evaluate which parameters vary in time, (iii) examine temporal characteristics of historical recruitment productivity, and (iv) evaluate productivity correlation across stocks. For 22 out of 28 stocks, at least one of the three time-varying parameter models had a better fit than the time-invariant model. In the CSE, fish productivity has diverse temporal patterns, with some stocks displaying relevant long-term decreasing productivity trends. Getting insight into temporal changes in recruitment productivity is very valuable and has important implications for sustainable fisheries.

Combining output control and fishing pressure limitations improves the management of the Japanese sardine Sardinops melanostictus

The stock biomass of the Japanese sardine Sardinops melanostictus in the Pacific Ocean shows large fluctuations in response to environmental variations and fishing activities; this is reflected in annual catch amounts. Harvest policy in Japan determines the current allowable biological catch (ABC) by using catch data and marine resource indices. The total allowable catch (TAC) for sardine is based on the ABC, but there is a time delay from the collection and analysis of data to its implementation. We have developed a new harvest control rule (HCR) into which fishing pressure limitations are incorporated. The rule calls for fishing to stop when fish landings reach the TAC or the fishing effort and the fishery catchability reach the limit. We have evaluated the performance of the proposed HCR against the implementation error for fishing pressure and the observation error for fishing mortality. We conducted simulations using two stock recruitment relationships and three historic starting years to investigate the effects of rule initiation timing. The results indicate that this HCR could help to facilitate a recovery of the stock biomass and improve annual catches of the Japanese sardine. Especially when the observation error is large, the HCR is effective in maintaining stock biomass, increasing the average annual catch, and stabilizing the overall annual catch.

Перспективи промислового використання карася Київського водосховища

Purpose. Establishing modern parameters of Prussian carp population as a promising object for increasing commercial fish productivity of the Kyiv Reservoir. Methodology. The work is based on field materials obtained during the period 2019-2021 on the Kyiv Reservoir. Data collection and processing was carried out in accordance with generally accepted methods. Ichthyological material was collected from catches of the survey gill nets with mesh sizes of 30-120 mm and commercial gill nets with mesh sizes of 38-50 and 75-100 mm. A total of 1215 net-days of surevy and commercial nets were analyzed during the study period, of which 2372 specimens of Prussian carp were analyzed. Statistical data processing was carried out using MS Excel. Findings. The main parameters that characterize the commercial stock of Prussian carp of the Kyiv Reservoir in the context of optimizing the commercial fishing pressure on this species have been determined. It was found that the state of recruitment of the commercial stock of this species was satisfactory and the dynamics of its size-age structure indicated a low commercial fishing pressure, especially on the right wing of the variation series — the average annual fishing mortality in 2019-21 was φM=0.13. The majority of the commercial stock is steadily composed of age-6 to age-10 fish, the share of older age groups was consistently high, which led to fluctuations in the weighted average age in catches within 6.7...8.5 years. It was shown that the intensification of Prussian carp harvest (due to specialized fishing with nets with mesh size of 50 mm) would allow increasing catches in the Kyiv Reservoir by 52...65%. Originality. For the first time, a detailed description of Prussian carp of the Kyiv Reservoir is presented as an adventive species that quickly assimilates free econiches and requires scientifically based measures to localize its abundance. Practical value. Based on the processing of information on the state of stocks of Prussian carp as part of the source of commercial fishery of the Kyiv Reservoir, the procedure of intensification of its commercial exploitation was determined, taking into account both fishery and environmental aspects. Proposals regarding the quantitative characteristics of specialized fishery with nets with mesh sizes of 50-60 mm are included in the new edition of the Rules for commercial fishery in inland fisheries water bodies (parts thereof). Key words: Kyiv Reservoir, Prussian carp, size-age structure of the catch, commercial stock, specialized fishing.

Social and ecological impacts, challenges, and lessons learned from the sandfish sea ranching experience in Maliwaliw Island, Eastern Samar, Philippines

Since the 1970s, Samar and Leyte islands had a thriving sea cucumber fishery but have rapidly declined over the years. In Maliwaliw Island, anecdotal accounts validated by the community revealed that the island had a brisk sea cucumber fishery but collapsed due to uncontrolled collection. In 2013, a sandfish sea ranching project was implemented in Maliwaliw Island, Salcedo, Eastern Samar, to bring back the sandfish that has disappeared from the area, and subsequently provide a supplemental livelihood to the local fishers. This study was conducted to investigate the impacts and lessons learned from the sandfish project. Recruitment of wild stock, increased community knowledge of sea cucumber biology and ecology, additional income through sandfish bycatch, and perceived enhancement of the coastal marine environment of the island were among the positive gains from the project identified by the respondents. On the other hand, poaching by fishers from neighboring communities resulting in inter-community conflicts was a major challenge. Clearly, the sandfish sea ranching project in Maliwaliw achieved ecological, social, and economic impacts on the project participants and the community. However, these gains need to be strengthened with effective management measures including size regulation to sustain the sea cucumber fishery in the area.

Impacts of predation by Eurasian otters on Atlantic salmon in two Norwegian rivers

Abstract The return of the Eurasian otter (Lutra lutra) to western Norway has sparked human–predator conflicts because otters prey on vulnerable Atlantic salmon (Salmo salar) populations. Although predation may not be the ultimate cause of salmon population declines, otters that kill adult salmon in rivers before they spawn impact the salmon spawning stock, with potential consequences for stock recruitment and ecosystem services (especially fisheries). To gain insight into impacts of otter predation on salmon populations, we quantified the predation by otters on adult salmon in two rivers in western Norway using a combination of radiotelemetry and temperature loggers. We tagged 30 salmon in Aureelva and 30 salmon in Søre Vartdalselva, and tracked the salmon until they died or left the river. This method identified the fates of 95% of tagged salmon. Estimated predation rates on adult salmon were 32% in Aureelva and 95% in Søre Vartdalselva. The salmon stock in Søre Vartdalselva was well below the spawning target, partly attributable to putatively additive mortality from predation by otters. Notwithstanding, we found no evidence that otters selectively killed salmon based on sex, length, health status, or activity level. Salmon in Søre Vartdalselva had greater predation risk compared to salmon in Aureelva, possibly due to differences in habitat types such as availability of holding pools. The presence of more holding area in Aureelva probably provided predation refuges for adult salmon that buffered the effectiveness of otter predation. Our findings emphasise that management decisions should be guided by river‐specific evaluations of impacts of predation on salmon. Otters are a very visible predator operating at the final phase of the life cycle before spawning, so predation is liable to be controversial given that salmon are now Red Listed in Norway.

Stage‐specific drivers of Pacific hake (Merluccius productus) recruitment in the California Current Ecosystem

AbstractUnderstanding environmental drivers of recruitment variability in marine fishes remains an important challenge in fish ecology and fisheries management. We developed a conceptual life‐history model for Pacific hake (Merluccius productus) along the west coast of the United States and Canada to generate stage‐specific and spatiotemporally‐specific hypotheses regarding the oceanographic and biological variables that likely influence their recruitment. Our model included seven life stages from pre‐spawning female conditioning through pelagic juvenile recruitment (age‐0 fish) for the coastal Pacific hake stock. Model‐estimated log recruitment deviations from the 2020 hake assessment were used as the dependent variable, with predictor variables drawn primarily from a regional ocean reanalysis for the California Current Ecosystem. Indices of prey and predator abundance were also included in our analysis, as were predictors of local‐ and basin‐scale climate. Five variables explained 59% of the recruitment variability not accounted for by the stock–recruitment relationship in the hake assessment. Recruitment deviations were negatively correlated with May–September eddy kinetic energy between 34.5° and 42.5°N, the North Pacific Current Bifurcation Index, and Pacific herring (Clupea pallasii) biomass during the spawner preconditioning stage, alongshore transport during the yolk‐sac larval stage, and the number of days between storm events during the first‐feeding larval stage. Other important predictors included upwelling strength during the preconditioning stage, the number of calm periods during the first‐feeding larval stage, and age‐1 hake predation on age‐0 pelagic juveniles. These findings suggest that multiple mechanisms affect Pacific hake survival across different life stages, leading to variability in population‐level recruitment.

«Unfaithful» Pink salmon. Random or regularity?

The reasons of «failed» forecasts for catch of pink salmon at the mainland coast of the Tatar Strait are analyzed. The classical «stock–recruitment» models produce regular errors in the forecasts of runs of pacific salmon under changing conditions of reproduction. To reduce the prediction errors, these models should include parameters of meteorological conditions in the freshwater period of salmon life. In particular case of pink salmon in the Tatar Strait, a relationship is revealed between the year-class strength and precipitation in winter during the period of egg incubation. Including this factor to the model based on Ricker equation increases the proportion of certainty in the variance of recruitment by more than 30 %.

What drives the recruitment of European sardine in Atlanto-Iberian waters (SW Europe)? Insights from a 22-year analysis

The European sardine (Sardina pilchardus) is the most abundant and socio-economically important small pelagic fish species in Western Iberia Upwelling Ecosystem. As a result of a long series of low recruitments, sardine biomass off Western Iberia has greatly reduced since the 2000s. Recruitment of small pelagic fish is mainly dependent on environmental factors. In order to identify the key drivers of sardine recruitment, it is essential to understand its temporal and spatial variability. To achieve this goal, a comprehensive suite of atmospheric, oceanographic, and biological variables spanning 1998–2020 (22 years) were extracted from satellite datasets. These were then related to in situ recruitment estimates obtained from yearly spring acoustic surveys carried out along two different hotspots of sardine recruitment of the southern Iberian sardine stock (NW Portugal and Gulf of Cadiz). Sardine recruitment in Atlanto-Iberian waters appears to be driven by distinct combinations of environmental factors, although sea surface temperature was identified as the main driver in both regions. Physical conditions that favour larval feeding and retention, such as shallower mixed layers and onshore transport, were also seen to play a vital role in modulating sardine recruitment. Furthermore, high sardine recruitment in NW Iberia was associated with optimal conditions in the winter (January–February). In contrast, recruitment strength of sardine off the Gulf of Cadiz were associated with the optimal conditions during late autumn and spring. The results from this work provide valuable insights to further understand the dynamics of sardine off Iberia, with potential to contribute to the sustainable management of sardine stocks in Atlanto-Iberian waters, particularly under climate change.

Evaluation of management performance of a new state-space model for pink salmon (Oncorhynchus gorbuscha) stock–recruitment analysis

A new state space stock –recruitment (SR) model (XSR) was developed to treat observation errors in spawner and catch data, and nonstationarity in productivity for pink salmon ( Oncorhynchus gorbuscha) . Closed loop simulation was used to evaluate the management performance of XSR and compare its performance with that of a traditional SR model (TSR) and a Kalman Filter (KF). XSR produced higher expected catch than TSR and KF over a wide range of conditions. In some situations, TSR was preferred for reducing conservation concerns. However, large “outcome uncertainty” (OU) or implementation error in achieving desired management objectives can substantially reduce the management benefits of all assessment methods. Thus, OU should be considered in all management strategy evaluations, otherwise results will be misleading. Accordingly, reducing OU may better help achieve objectives.

Multidecadal fluctuations in green turtle hatchling production related to climate variability

The state of Campeche, Mexico, harbors one of the largest green turtle (Chelonia mydas) rookeries of the Wider Caribbean Region. Since the 1970s, harvesting of this population was common practice, but it has since ceased, and the population is rebounding as a consequence. In this rookery, during the past 37 years (1984–2020), the positive relationship between the annual number of nesting females and the number of hatchlings they produce has revealed a long-term population signal that we postulate could be related to environmental factors. To investigate this relationship more deeply, we adopt a stock-recruitment (SR) approach, which is commonly used in fisheries. Regression analysis methods for the SR relationship, including a dynamic version of the model that incorporates the effect of sea surface temperature, show that the number of recruits produced and the number of hatchlings per unit nester were significantly and inversely correlated with a 26-year cycle of the Atlantic Multidecadal Oscillation (AMO) with a three year lag. A possible explanation for this finding is that environmental conditions during warming periods of the 26-year AMO cycle may negatively affect hatchling production by altering the nest moisture content during the incubation period, and increasing embryonic mortality, while the annual female abundance at nesting beaches may decrease due to trophic effects. The time series of abundance corresponding to other population units of green turtles as well as other species of sea turtles in the Gulf of Mexico present a similar behavior to that evaluated here, suggesting a basin-wide environmental effect.

Contrasting distributional patterns of two co-occurring ctenophores in relation to ichthyoplankton and environmental features in the Southwestern Atlantic Ocean

Assessing the ecological role of ctenophores is crucial to understanding ecosystem functioning, including the recruitment of fish stocks. During austral summer in the North Patagonia Ecosystem (NPE) (41-48°S), the ctenophores Mnemiopsis leidyi and Pleurobrachia pileus co-occur with the peak of ichthyoplankton abundance of 2 important fishing resources: the Argentine anchovy Engraulis anchoita and the Argentine hake Merluccius hubbsi, in association with a highly productive tidal front system. We analyzed 367 plankton samples collected in the NPE over 13 yr (2005-2018) to determine the abundance and spatial distribution of M. leidyi and P. pileus, their spatial overlap with ichthyoplankton (eggs and larvae of hake and anchovy), and their relationship with environmental features. Both ctenophore species displayed contrasting distributional patterns, associated with different physical environments, particularly temperature. M. leidyi showed a higher spatial overlap with ichthyoplankton compared to P. pileus. In addition, we found a significant negative correlation between the biomass of M. leidyi and the abundance of both fish larvae, suggesting a potential negative impact (via predation and competition) on the survival and recruitment of these 2 fisheries resources.