Daily Egg Production Method Research Articles

A Bayesian spatially explicit estimation of daily egg production: application to anchovy in the Bay of Biscay

Biomass estimates of fish resources by the daily egg production method (DEPM) are sensitive to the high variability of the daily egg production ( P0) and egg mortality (Z) in space. This work presents a Bayesian approach to estimate these parameters. A prior distribution of Z based on literature serves to overcome the biologically implausible Z estimates that can result from frequentist approaches. In addition to the classical estimation of a single P0 over the spawning area, the Bayesian framework allows also the modelling of egg densities in space, by including either spatial random effects, smoothing functions, or kriging like models, providing insights into the spatial variability of P0. The Bayesian approach was applied to the Bay of Biscay anchovy DEPM surveys. Results showed that this Bayesian approximation solved the implausible Z problem resulting in tighter credible intervals of both P0 and Z. Overall, spatial models outperformed the non-spatial model in terms of goodness of fit and resulted in slightly different total production estimates across models for each year, with a moderate decrease on uncertainty estimates.

Large sardine resource discovered off south-eastern Australia: Potential risks, challenges and benefits of establishing a new fishery

This paper documents the discovery of a large sardine (Sardinops sagax) resource off South-eastern Australia. We use information from three ichthyoplankton surveys for jack mackerel to apply the Daily Egg Production Method (DEPM) to the South-eastern sardine stock. Mean daily egg production was estimated from survey data. Spawning area was estimated from the surveys and using a habitat suitability model to predict presence/absence of eggs in unsurveyed areas. Spawning biomass was calculated using adult parameters from the adjacent Southern stock. The spawning biomass in the area surveyed was 262,564 t (95% CI 125,670–326–438 t). Habitat modelling using a conservative threshold suggested that the total spawning biomass was 346,388 t (95% CI 143,936–548,840 t). These results suggest this stock has the capacity to sustain an annual catch of several tens of thousands of tonnes, which is considerably larger than the recent catch of < 2500 t. Our findings provide further evidence that opportunities exist to increase Australia’s fisheries production. Risks and challenges associated with establishing a new large-scale sardine fishery, including bycatch and ecological impacts, and opposition from conservation groups and recreational fishers are discussed. We also outline the potential socio-economic and environmental benefits, including the use of marine protein and oil from a sustainable local source with low environmental impacts as an input to the AU$1.3B Tasmanian salmon aquaculture industry. We emphasize the need to adopt a precautionary approach to developing a new fishery and the critical importance of conducting comprehensive surveys to confirm our findings and inform ongoing management.

Expansion and contraction of the Engraulis ringens spawning area in northern Chile

Many studies have considered the temporal patterns of reproduction of Engraulis ringens, but little attention has been given to the spatial patterns of reproduction (spawning areas), which have shown great variability in both location (geographical position) and extent (the area covered by positive stations), without a satisfactory explanation. Along the Chilean coast, the daily egg production method (DEPM) has been used for several years to estimate spawning biomass in the northern Chile E. ringens population, with 24 surveys being carried out from 1992 to 2020. The most fluctuating parameters estimated in the DEPM have been the location and extent of the spawning area and the average female weight. In the last few years the eggs were distributed near the coast, whereas in the first few years they showed a more oceanic distribution. The average female weight has fallen from 30 g in the 1990s to 11 g in the last few years. In the present study, we analysed the relationship of the location and extent of spawning areas with female size and abundancy in E. ringens from northern Chile. According to the results, periods of high abundancy of eggs are positively correlated with larger females that spawn in more oceanic zones and to the south, expanding the spatial egg distribution and increasing the egg density. By contrast, in years with low abundance of eggs, the spawning was located north and towards the coast, with a smaller spawning area and smaller female size.

Empirical survey-based harvest control rules in a transboundary small pelagic fishery under recruitment regime shifts: The case of the northern Chilean-southern Peruvian anchovy

The management of small pelagic fisheries is a complex task due to its large variability of both environmental and biological processes. This high variability can impact the reliability of stock assessment models and fishery management recommendations. When this occurs, management decisions should explore alternative control rules based on empirical abundance indices or pragmatic management procedures. In this paper, we examined the performance of a harvest control rule (HCR) for the Chilean fleet in the shared Chile-Peru anchovy (Engraulis ringens) fishery (16°S–24°S), which is one of the most important stocks in the Humboldt marine ecosystem in the Southeast Pacific. The HCR uses the annual changes in spawning biomass estimated by Daily Egg Production Method (DEPM) surveys in the waters off northern Chile. Analyzes are carried out through operating models simulating population regime shifts under different uncertainty sources. The results show that despite the latent risk of overfishing caused both by recruitment variability and the lack of fisheries management coordination between two jurisdictions, the control of one of the parties' fishing mortality can enable the sustainability of the entire stock, provided the fishing effort of the other jurisdiction be remained at similar levels as in the last decade. The use of an HCR would avoid increases in fishing mortality and would facilitate BMSY-based management goals. This research determined conditions under which the shared Chile-Peru anchovy fishery can be sustainable considering different sources of uncertainty of the biological-fishing system.

Generalizing the Parker equation of DEPM: Incorporating the size dependence of population number and reproductive inputs to estimate spawning biomass and female population by size

Many fisheries assessed using the daily egg production method (DEPM) have data available that inform the dependence of adult population structure and fecundity on fish body size. Yet most do not consider this size dependence and use means of female body weight and batch fecundity. Here we extend the Parker (1980) equation for DEPM spawning biomass to make explicit the size structure of females. Our generalized DEPM approach assumes female body size partitioned into weight classes. It uses data inputs of female population number proportions and batch fecundity by discrete weight class or a continuous fitted relationship of batch fecundity versus weight. We derive closed-form estimation formulas for total spawning biomass, and for two new DEPM outputs, total female population number and female population numbers by weight class. We also derive analytic formulas for the standard errors of these three population abundance estimates using the delta method. The allometric dependence of batch fecundity on female body weight is fitted by maximum likelihood with allometric error structure. In fishery management application, a principal benefit of this size-based generalization of DEPM is the added output of absolute female population numbers by weight class, which can be fitted in a fishery assessment model or can directly inform management decision making. We have posted an R package “DEPM” containing these new estimation functions and their delta method standard errors. We apply this size-based DEPM method to South Australian snapper, Chrysophrys auratus, for which large estimated decreases in spawning biomass from 2014 to 2018 informed a strong management response.

Increasing the precision of the daily egg production method; 2020’s remix of a 1980’s classic

Abstract This paper reviews application of the Daily Egg Production Method (DEPM) to sardine (Sardinops sagax) off southern Australia between 1995 and 2019. Coefficients of variation (CVs) of estimates of spawning biomass (SB) were reduced from 23–59% to 8–12% by: (i) estimating mean daily egg production (P0), spawning fraction (S), and sex ratio (R) from all historical data rather than annually; and (ii) combining batch fecundity (F) and female weight (W) into a single parameter, relative fecundity (F′ = F^/W). Total daily egg production was estimated most precisely from annual estimates of spawning area (A) and estimates of P0 obtained from historical data. Both S and R were estimated most precisely from historical data. Estimating W and F from historical data did not increase precision. F′ had lower CVs than both W and F, and was stable across years and a wide range of W. Findings demonstrate that A can be converted into a precise estimates of SB using estimates of P0, S, R, and F′ obtained from historical data. However, the possibility that DEPM parameters may change in the future cannot be discounted. Future monitoring should include annual estimation of P0 and periodic (e.g. 3–5 years) re-estimation of adult parameters.

Contrasting post-ovulatory follicle production in fishes with different spawning dynamics

The assessment of postovulatory follicles (POFs) is of considerable importance when applying the Daily Egg Production Method (DEPM) since it is used to estimate both spawning frequency (S) and batch fecundity (FB), provided that daily cohorts of POFs are discernible. Atlantic sardine (Sardina pilchardus) and Atlantic mackerel (Scomber scombrus), two commercially valuable pelagic species with different spawning dynamics were investigated principally to evaluate the appropriateness of the POF method to properly reflect S and FB. For appropriate quantification, both the Weibel and the postovulatory follicles packing density (POFPD) method, the latter described in this paper, were applied and compared. In sardine, not only was the existence of one single daily POF cohort confirmed but also the estimated number of POFs in the cohort matched gravimetric FB results. Furthermore, the Weibel method and POFPD led to similar results. However, several daily POF cohorts co-occurred in the mackerel ovary. Therefore, POF-based estimation of FB only works for species such as sardine with a single, daily POF cohort. Likewise, in relation to estimation of S, the occurrence of single versus multiple POF cohorts makes the histological assignment of spawners to daily spawning classes either straightforward or difficult for species comparable to sardine or mackerel, respectively. In the latter case we suggest focusing in the largest, thus newest, POFs which correspond to the latest spawning event. Therefore, the appropriateness of the POF method in applications of the DEPM should be judged based on the spawning dynamics of the assessed species.

Spawning ecology of the European anchovy (Engraulis encrasicolus) in the Strait of Sicily: Linking variations of zooplankton prey, fish density, growth, and reproduction in an upwelling system

A 12-year time series (2005–2016) was examined to explore relationships between European anchovy density, growth, reproduction and habitat dynamics in an upwelling system. Specifically, data used for a daily egg production method were combined with oceanographic data, prey availability, as well as acoustic surveys of the anchovy stock in the Strait of Sicily, in the Central Mediterranean Sea. Variables typically used for evaluating habitat dynamics (i.e. water temperature, chlorophyll-a, zooplankton concentration, kinetic energy, mixed layer depth and upwelling index) indicated strong upwelling events during the summer, when anchovy spawns. A linear ‘principal component’ combination of environmental traits, associated with summer upwelling, was identified by step-wise regression as a driver for growth (length at the end of the first year; L1), reproductive investment (gonad-somatic index; GSI), as well as egg production (daily specific fecundity; DSF). These relationships are consistent with direct energy flow from the environment to both somatic growth and reproductive output, indicating an income breeding strategy by anchovy (i.e. surplus energy acquired during the summer breeding season is used directly for reproductive growth). Step-wise regression also identified three additional relationships: (1) a density-dependent mechanism reducing growth (L1) and fecundity (DSF) at higher fish densities; (2) higher fish condition (Kn) positively affecting growth (L1); (3) higher prey availability (mesozooplankton concentration) positively affecting GSI. This time series and approach are promising for exploring the abiotic and biotic mechanisms setting year class strength in advance of recruitment to the fishery.

Daily anchovy (Engraulis ringens) egg mortality rates in different spawning zones along the Chilean coast

The daily egg mortality (Z) of small pelagic fish can be modulated by environmental variables as well as by population derived factors such as egg production and/or dispersion and aggregation processes. The anchovy daily egg mortality rate was compared across spawning areas distributed in the North (18∘25’S-26º03’S), Central (34º30’S-37º10’S), and South (38∘S-41∘20’S) zones along the Chilean coast. Daily egg production (P0) and Z were estimated through a generalized linear model (binomial negative) from egg count data obtained in yearly Daily Egg Production Method applications (2002–2014). Co-variables included P0, the spawning area (SA), sea surface temperature (SST), phytoplankton biomass (Chl-a), alongshore wind (V) and an index of the El Niño-Southern Oscillation (ENSO) during the spawning period (August–September).Egg mortality was significantly higher in the North zone, influencing spatial differences rather than yearly effects. Egg mortality was not associated with ENSO, but correlated positively with P0, SA, SST, and V, and negatively with Chl-a. The best model explained 42.3% (P<0.01) of the egg mortality rates and included SST and Chl-a. In the South zone, the egg mortality was significantly correlated with P0 and V, which was interpreted as density-dependent effects and winds driving egg concentration processes rather than dispersion. The Lloyd’s patchiness index was computed as an index of egg aggregation by embryonic development stages (Group A: early development, Group B: middle development, and Group C: close to hatching). The spawning area was negatively correlated with the patchiness index of Groups A and B in the North zone. Overall results suggested that anchovy eggs survival would be dependent on dispersion in the North zone but concentration and retention in the South spawning area and thereafter coincident with reproductive tactics of adults in the North and South areas, respectively.

Spawning marker patterns of Engraulis ringens of northern Chile

The procedures used to estimate the daily spawning fraction of northern Chile anchovy (Engraulis ringens) have been revised based on information from 17 Daily Egg Production Method (DEPM) survey estimates of the anchovy spawning biomass. The most commonly used spawning marker is postovulatory follicles (POFs). POFs are transitory remnants of the ovulated follicles that remain within the ovary after spawning, until they are fully resorbed. As the process of Postovulatory Follicles (POF) degeneration is continuous, the assignment of POF stages to daily cohorts of spawners can be imprecise. Therefore, the main objective of this paper is to review the criteria used to assign females to daily spawning cohorts in anchovy (Engraulis ringens) located off northern Chile. For our first step, we review the peak spawning time using recently fertilized eggs. To study the POF duration, we divided the POF degenerative process into seven stages and then fitted a gamma probability distribution to the hourly POF stage fractions in a daily cycle. This approach is a quantitative, objective way to define the range of time during which the POF stages are present in the samples, as well as the overlap between daily cycles. The hourly distribution of recently fertilized eggs (Stage 1) indicated a strong daily spawning time synchrony that peaked at 4:00 GMT (00:00 local time). We found that the class Day1 cohort (females that spawn on the night before their capture) belongs primarily to POF Stages 1 to 3, with some proportions belonging to POF Stages 4 and 5 according to the capture time. The migratory nucleus (MN) and hydrated (H) phases correspond to the cohort that spawns on the night of capture. The daily spawning fraction estimates the based on the POF or MN and H phases did not show significant differences. Our results indicated an average spawning frequency of four days (S = 0.27 day−1). The prevalence of POFs according to female size follow an asymptotic model of the von Bertalanffy type, indicating an asymptote around 0.27 day−1. The POF presence was higher during cold periods, probably due to a longer period of decay. A general linear model analysis showed that body weight, time of catch and mean temperature significantly influenced the spawning fraction.

DNA barcoding reveals clear delineation between spawning sites for neritic versus oceanic fishes in the Gulf of Mexico

AbstractWe combined research‐vessel cruises of opportunity with DNA barcoding to survey planktonic, percomorph fish eggs at 40 stations distributed across and around the Gulf of Mexico (GoM). The objectives were (a) to determine whether eggs of fishes that are potential candidates for the daily egg production method (DEPM) can be readily barcoded, (b) to identify taxa that are spawning in the GoM, (c) to determine encounter rates for eggs of economically valuable taxa, and (d) to characterize individual egg taxa as being primarily neritic, primarily oceanic, or primarily mixed (i.e., both neritic and oceanic). Of the 1,144 eggs that were individually barcoded, 709 (62%) were definitively identified to species (62 species from 42 families), with an additional 20 taxa identified to genus or subfamily level. The eggs of 15 economically important species were identified, most of which had intermediate encounter rates and moderately dispersed spatial distributions, as indicated by an index of aggregation. SIMPROF analysis of stationwise cluster analysis identified eight significant groups within the 35 stations that yielded percomorph eggs; a corresponding specieswise analysis identified six groups of stations as having a neritic egg community and two groups as having an oceanic community, with a community transition located at the shelf break. Although the neritic and oceanic stations did not share important species, it remains possible that coastal pelagic species have mixed neritic–oceanic distributions. Together, these results indicate DEPM fish‐egg surveys based on DNA barcoding are feasible at the large marine ecosystem scale.

Estimation of the spawning biomass of myctophids based on larval production and reproductive parameters: the case study of Benthosema pterotum in the East China Sea

AbstractThis study estimated the spawning biomass of a myctophid by applying the daily egg production method (DEPM) based on data of larval fish surveys and reproductive parameters. Benthosema pterotum in the central part of the East China Sea shelf was used as the model species, as ecological and reproductive data are available in the literature. This study used data of the larvae and adults sampled in late summer when the primary spawning occurs. Daily egg production was estimated by back-projection of the daily production of larvae at hatching by 10 h, assuming that the mortality rate during the egg stage is the same to that of the larval stage. This study determined the sex ratio, batch fecundity, and spawning fraction. As a result, spawning biomass of B. pterotum in the East China Sea shelf was estimated to be 9036 tons. The study also assesses and discusses several sources of potential uncertainty. The relative sensitivity of estimates of spawning biomass to variations in each parameter showed a four fold difference between the lowest and highest estimates (4066–16 265 tons). Since this was comparable to the biomass estimated by a swept-area trawl survey, the approximate estimation of biomass would be possible by applying this method. Considering that larval fish surveys have been conducted in the world’s oceans and myctophids have always dominated in the samples, application of the DEPM is a potential option for estimating the order of magnitude of the biomass of myctophids.

Atlantic mackerel daily spawning dynamics and implications for batch fecundity estimations

Abstract The present study contributes to a better understanding of the daily spawning dynamics of southern NEA mackerel (Scomber scombrus) with implications for the estimation of batch fecundity. It shows that there is a time window during the day, mainly in the afternoon, during which the advanced oocyte mode in imminent spawners separates from the remaining, smaller oocytes. This synchronicity in the separation of the spawning batch amongst imminent spawners corroborates evidence for the existence of daily spawning synchronicity in the population. This is particularly important for applications of the daily egg production method, DEPM, because such pattern facilitates both the ageing of eggs for the estimation of the daily egg production at sea and the ageing of postovulatory follicles for the estimation of spawning frequency. For NEA mackerel, batch fecundity could only be measured when a clear hiatus was established between the spawning batch and the smaller oocytes. Hydrated females that do not show such hiatus would not be valid for batch fecundity measurements suggesting that the “hydrated oocytes method” is not fully applicable for this stock. Knowing the time of day at which the batch is separated, will facilitate the sampling of valid females for the estimation of batch fecundity.

Preovulatory follicle growth dynamics indicates daily spawning synchronicity in wild-caught Atlantic horse mackerel off Portugal

The Atlantic horse mackerel, Trachurus trachurus, is a commercially important species whose daily spawning dynamics are not yet fully understood. Existence of spawning synchronicity would facilitate application of the daily egg production method for estimating the spawning biomass of the southern stock of the species in the Northeast Atlantic. The intent of this work was to study preovulatory follicle (PREF) growth dynamics in wild Atlantic horse mackerel to determine the existence of daily spawning synchronicity in the population. The size and translucency of hydrated oocytes increased significantly between early morning and late afternoon; this pattern was the same for two study years. This morpho-developmental shift led to the appearance of previously indistinguishable oil drops in ovarian whole mounts which is an easily observed microscopic descriptor of PREF growth. The results clearly suggested that PREF growth was well synchronized between Atlantic horse mackerel that were capable of spawning and provided evidence for the existence of daily spawning synchronicity in the population. Apart from sampling time of day, the size of hydrated oocytes was shown to be affected by body size: a 10 cm difference in maternal length corresponded to a 0.030 mm difference in the diameter of hydrated oocytes. In contrast, the size of early germinal vesicle migration oocytes was correlated neither to sampling time nor to maternal size which suggests that the abrupt increase in PREF size only commences after the onset of the actively spawning sub-phase.

DEVELOPMENT STAGE, AGEING AND NATURAL MORTALITY OF ANCHOVY EGGS (Encrasicholina SPECIES) FROM BACAN

ln order to study the possible application of the Daily Egg Production Method (DEPM) for estimating anchovy (Encrasicholina spp.) biomass at Bacan. Maluku, we collected a large series of plankton samples.

Characterising the spawning patterns of Jack Mackerel (Trachurus declivis) off eastern Australia to optimise future survey design

Estimates of spawning biomass obtained using the daily egg production method (DEPM) are used to establish catch limits for Jack Mackerel (Trachurus declivis) off eastern Australia. Information from concurrent ichthyoplankton and adult surveys conducted between Port Stephens, New South Wales and South East Cape, Tasmania during January 2014 was used to assess the environmental factors that determine the spawning patterns of Jack Mackerel. Adults were collected using a modified demersal trawl net deployed during daylight hours. Samples were unbiased with respect to sex, spawning activity and size. Large fish were collected from both the inner shelf and shelf break; spawning fractions and egg densities were high inshore and low offshore. These findings suggest complex spatio-temporal patterns of spawning, different to previous studies suggesting that most spawning occurred at the shelf break (∼200m). Eggs were most abundant in sea surface temperatures (SSTs) of 15–20°C and at depths of <130m. Future ichthyoplankton surveys should target waters with SSTs of 14–23°C and depths of 30–250m. Future adult surveys should sample the same range of depths and latitudes as the ichthyoplankton surveys and be structured as systematically as permitted by the availability of habitats suitable for demersal trawling. The DEPM does not provide information about the abundance of non-spawning adults outside the main spawning area. Extending future adult surveys beyond the spawning area would address this limitation by providing estimates of the distribution and relative abundance of adults across the entire range of the population. Findings of this study will help to improve the design of future DEPM surveys.

Spawning biomass of jack mackerel (Trachurus declivis) off eastern Australia: Critical knowledge for managing a controversial fishery

Jack mackerel (Trachurus declivis) is a key target species of the Australian Small Pelagic Fishery. The fishery was subject to extensive public scrutiny following an attempt to introduce a large factory-trawler in 2012. One of the concerns raised related to the reliability and age of estimates of the spawning biomass of jack mackerel. This study reports on the first dedicated application of the Daily Egg Production Method (DEPM) to jack mackerel. Large numbers of samples of eggs and adults were collected concurrently from the key spawning area off eastern Australia during what has been previously identified as the main spawning period. This study established an effective adult sampling method and provides the first estimates for this species of adult reproductive parameters required for application of the DEPM. Estimates of all DEPM parameters, except batch fecundity, were based on relatively large sample sizes. The spawning biomass of jack mackerel off eastern Australia during January 2014 was estimated to be approximately 157,805 t (95% CI=59,570–358,731 t). Sensitivity analyses indicate this estimate is robust to likely uncertainty in estimates of key parameters. Future research should focus on elucidating the size-fecundity relationship and spatial, temporal and size-related variations in spawning fraction.

Assessing natural mortality of Bay of Biscay anchovy from survey population and biomass estimates

The closure of the anchovy (Engraulis encrasicolus) fishery in the Bay of Biscay between 2005 and 2010 because of low biomass levels provided an opportunity to estimate natural mortality using data from egg (daily egg production method, DEPM) and acoustic surveys implemented for the assessment of this population since 1987. Assuming that natural mortality (M) is constant over time and that catchability in both surveys is equal for all ages, M could be estimated using log-linear models on the series of surveys of population numbers at age and seasonal integrated stock assessments. The analysis suggests M values of around 0.9 for a common natural mortality at all ages. However, we found firm evidence that natural mortality at ages 2 and older (M2+) is markedly higher than at age 1 (M1), which indicates senescent mortality, a possibility suggested a long time ago for this type of short-lived species.

Recent collapse of northern anchovy biomass off California

New estimates of abundance of northern anchovy (Engraulis mordax) are developed from California Cooperative Oceanic Fisheries Investigations (CalCOFI) data on egg and larval densities for the period 1951–2011. Previous estimates utilizing mean density over the CalCOFI area show a hyperstability bias because of the nearshore concentration of CalCOFI stations and the tendency of the anchovy population to contract into this area when abundances are low. New abundance estimates weight sample egg and larval densities to the local area represented by that sample, and then sum contributions to obtain total abundance. We develop total egg and larval abundance estimates for January and April, form a combined index, and calibrate it to the absolute biomass estimates produced by the Daily Egg Production Method in the early 1980s. Anchovy spawning biomass was very low, 10,000–20,000 Metric Tons (MT), in the early 1950s when CalCOFI sampling began. Abundance increased and fluctuated between 0.5 and 2 million metric tons from 1960 through 1990. After 1990, spawning biomass fluctuated around 200,000 tons, briefly increased during 2005–2006, then declined substantially over four years to below 20,000 tons from 2009 to 2011. CalCOFI ichthyoplankton data collected after 2011 are not yet available for analysis, but continuous underway egg sampling conducted during CalCOFI cruises indicates continued low abundance and very limited spawning through 2015 in both southern and central California. The recent collapse of anchovy abundance occurred in the absence of a significant fishery. Present annual catches of a few thousand tons are small by historical comparison, but the exploitation rate may now be relatively high given the low stock abundance. The decline in anchovy abundance coincides with recent die-offs and reproductive failures of anchovy-dependent predators in the ecosystem (e.g., sea lions and pelicans).

Application of GAMs and multinomial models to assess the spawning pattern of fishes with daily spawning synchronicity: A case study in the European anchovy (Engraulis encrasicolus) in the central Mediterranean Sea

Information on the breeding behaviour of fish such as spatial segregation phenomena during spawning events is important both for understanding their reproductive ecology but also for correctly designing sampling schemes in stock assessment. The daily spawning pattern of the European anchovy, Engraulis encrasicolus, was assessed using a novel methodology that included the analysis of the hourly distribution of imminent and recent spawning stages using a combination of generalised additive models (GAMs) and of multinomial models for ordinal categorical data. To do so we used data from nine years of daily egg production method, DEPM, surveys, carried out in the Strait of Sicily. The spatial distribution and occurrence of actively spawning individuals were also analysed. Results showed that the actively spawning phase period (ASPP), i.e., the period from the onset of oocyte hydration to spawning ranged approximately 4.5h between ca. 18:30 and 23:00 while a daily spawning peak was identified at 22:30. The sex ratio was skewed during the ASPP such that females exceeded males by ca. 20% at mid ASPP. During the ASPP the proportion of Day-0 females (i.e., females that spawned or would spawn the day of sampling) decreases with increasing bottom depth while outside the ASPP Day-0 spawners did not show any particular bathymetric pattern. In the study area, anchovies exhibit segregative spawning behaviour which drives spawners to select shallower waters, similar to other anchovy populations in the Mediterranean while in the Atlantic and in other clupeoid populations the opposite bathymetric trend has been observed.