Sardine Recruitment Research Articles

Evaluating robustness of harvest control rules to climate-driven variability in Pacific sardine recruitment

Climate-driven changes in ocean temperatures, currents, or plankton dynamics may disrupt pelagic forage fish recruitment. Being responsive to such impacts enables fisheries management to ensure continued sustainable harvest of forage species. We conducted a management strategy evaluation to assess the robustness of current and alternative Pacific sardine harvest control rules under a variety of recruitment scenarios representing potential projections of future climate conditions in the California Current. The current environmentally informed control rule modifies the harvest rate for the northern sardine subpopulation based on average sea surface temperatures measured during California Cooperative Oceanic Fisheries Investigations field cruises. This rule prioritizes catch at intermediate biomass levels but may increase variability in catch and closure frequency compared to alternative control rules, especially if recruitment is unrelated to ocean temperatures. Fishing at maximum sustainable yield and using dynamically estimated reference points reduced the frequency of biomass falling below 150 000 mt by up to 17%, while using survey index-based biomass estimates resulted in a 14% higher risk of delayed fishery closure during stock declines than when using assessment-based estimates.

Impact of Drought and Wildfires in Recent Trends of Diarrhetic Shellfish Toxins in Cockles from Northwest Portugal and Its Similarities with Sardine Stock Trends in the Period 2001–2022

In Portugal, high levels of diarrhetic shellfish toxins (DSTs) originating from Dinophysis spp. are accumulated in bivalves, mainly on the northwest coast (NWC). The occurrence of DSTs in spring and early summer is positively related to precipitation. A decrease in average precipitation after 2003 led to a reduction in DSTs after 2008. However, the accumulation of DSTs in the NWC increased in the period 2020–2022. The hydrological year of 2022 was marked by extreme drought. In cockles from the endemic area of Ria de Aveiro, 37.3% of the weeks tested presented samples above the regulatory limit (RL). The previous record was 31.9% in 2005, also an extremely dry year. The average percentage of weeks above the RL in severe drought years surpassed both the low and high precipitation years. In severe dry years, toxicity was dominated mainly by D. acuta toxins. While D. acuminata grows after the abundant river discharges in spring, D. acuta is capable of growing in summer during upwelling favourable conditions. In the last two decades, extensive forest wildfires were recurrent, but the area burnt reduced sharply after 2017 following tighter fire control measures. Low levels of DSTs or low percentage of weeks with cockles above the RL were related to high burnt areas in the previous year. The recent increase in 2020–2022 of DSTs, a planktonic biomarker in bivalves, was also coincident with the similar temporal increase in either the recruitment or the biomass of Sardina pilchardus, a planktivorous fish in decline after 2005/2006. Both low sardine recruitment and biomass were coincident with low precipitation or high burnt areas. Wildfires degrade coastal water quality, as seems reflected both in the cockle’s DST and the sardine stock time series.

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.

River Runoff as a Major Driver of Anchovy (Engraulis ringens) Recruitment but Not of Common Sardine (Strangomera bentincki) in Central-South Chile

River runoff to the coastal zone is one of the most important environmental variables that influences the structure and functioning of the neritic trophic web and modulates temporal fluctuations of coastal fishery production in many ecosystems worldwide. The relationship between recruitment of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) in central-south Chile (34°–40°S) and fluctuations in river runoff was analyzed during the last two decades (1999–2018) using linear and nonlinear regression models. River runoff was also incorporated as a co-variate in Beverton-Holt (BH) stock-recruitment models. Anchovy recruitment was found to be positive and significantly associated with river runoff for all analyzed rivers both for the runoff calculated during its main reproductive period (July–October) and for each single month during this period. This detected nonlinear relationship remained significant when runoff was analyzed for the central macrozone (CMZ) and southern macrozone (SMZ). In contrast, no relationship between runoff and recruitment was detected for the common sardine in any river or macrozone (CMZ, SMZ), for each month of the main reproductive season or for the whole reproductive period (July–October). No density-dependent regulation was detected using BH stock-recruitment models for either species, with or without total freshwater input (i.e., the joint discharge of all rivers; RIVSUM index) as a covariate. Our results highlight the relevance of river runoff as a major driving factor in anchovy recruitment variability, suggesting that high (low) monthly runoff anomalies during the main reproductive period of anchovy populations markedly affect the survival of its early stages, altering recruitment success. For common sardine the runoff effect was not significant, which suggests that other oceanographic/climatic factors (wind-driven upwelling, sea surface temperature, El Niño) are more relevant in driving the variability of its recruitment than river runoff.

Insights into the environmental conditions contributing to variability in the larval recruitment of the tropical sardine Sardinella lemuru

• Biophysical model used to determine conditions experienced by sardine larvae • Larvae from peak spawning period experienced more optimal conditions for survival • Lower temperature and higher chlorophyll-a offset higher advective loss • Temporal variability in these conditions can affect sardine stocks • Current management plans need to account for environmental variability apart from fishing pressure The small pelagic fishery in the southern Philippines is one of the largest contributors to fisheries production in the country, and is dominated by the Bali sardine Sardinella lemuru . This species is a year-round spawner that has its peak spawning period during the northeast monsoon (NEM) months of November to February. However, there is still limited information on the conditions that affect this species’ survival during its early life history stages. Here, we attempt to discern the importance of temperature, prey density and advection on sardine larvae. The larvae were represented as passive particles that were released in known spawning grounds to simulate dispersal. The conditions the larvae experienced, namely, temperature and estimated prey density based on satellite chlorophyll-a values were recorded at each time step until the estimated recruitment age of 60 days. The temporal and spatial averaged conditions of recruited sardines showed that sardines spawned during the NEM months experienced higher chlorophyll-a, lower (more optimal) sea surface temperature, albeit higher advective loss, than sardines spawned during other months. Comparisons between years showed that during higher reported catch years, sardine larvae experienced lower temperatures and higher retention nearshore. Our results emphasize that sardine stock management efforts need to recognize the contribution of the temporally variable sardine environment to patterns in sardine recruitment and consequently in catches.

Revisiting sardine recruitment hypotheses: Egg‐production‐based survival index improves understanding of recruitment mechanisms of fish under climate variability

AbstractProportionality between spawning stock biomass (SSB) and annual total egg production (TEP) has been an inherent assumption in spawner–recruitment models for fisheries management and studies on recruitment mechanisms in fish. However, recent evidence of density dependence in egg production has led to a need to reconsider the validity of recruitment per spawning stock biomass (RPS), which is a SSB‐based survival index from hatching to recruitment, in recruitment studies. Here we revisit sardine recruitment hypotheses based on recruitment per egg production (RPE), which is a TEP‐based survival index, by reanalyzing published data for the Pacific stock of Japanese sardine (Sardinops melanostictus) in the Kuroshio Current system at a multidecadal scale. By adopting RPE, we detected statistically significant relationships of the survival from hatching to recruitment to large‐scale ocean climate anomaly, ambient temperature in the nursery grounds and growth rate during the early life stages. None of these relationships were detected by RPS. We propose the adoption of TEP‐based survival indices rather than SSB‐based ones in future studies on recruitment mechanisms of fish in relation to environmental and biological factors. Developing egg‐production‐based survival indices has the potential to improve understanding of recruitment mechanisms of fish under climate variability.

Investigating connectivity between two sardine stocks off South Africa using a high‐resolution IBM: Retention and transport success of sardine eggs

AbstractThis study applied a previously used Lagrangian individual‐based model (IBM) for sardine in the Southern Benguela to an improved and more robust hydrodynamic model to investigate whether a more representative spatial coverage, greater horizontal and vertical resolution, more realistic winds and improved representation of mesoscale features such as eddies and filaments would give different results for transport and retention of early life stages. Despite major differences between the old and new hydrodynamic models, overall the IBM results were quite similar to the previous southern Benguela sardine IBM study. This surprising result indicates that it is the macroscale circulation features resolved by the two hydrodynamic models that are controlling transport and retention of sardine early life stages. The contribution of transient mesoscale features such as eddies and filaments appears to be less important when transport patterns are averaged over the 21‐year‐long experiment. Another aim of this study was to better estimate the contribution of south coast spawning to west coast sardine recruitment. This was possible because of an eastward extension of the geographical domain of the new hydrodynamic model which provided a more realistic representation of the south coast spawning ground. Three main spawning and nursery area systems, similar to those identified in the previous sardine IBM, were identified: west coast and west coast (WC‐WC), south coast and west coast (SC‐WC), and south coast and south coast (SC‐SC). Spawning area proved to be an important determinant of modelled retention and transport success, with spawning depth also playing an important role on the west coast. The main difference observed from the previous study was an increase in the average percentage of particles released on the south coast and transported to the west coast (P0, 17.4%). This indicates more connectivity between the southern and western sardine stocks than previously thought and is therefore important for fishery management. Standardized anomalies from the modelled retention/transport were compared with recruitment estimates from stock assessment models but there was no correlation between the two sets of anomalies. However, a significant correlation was observed between the modelled retention/transport anomalies for the west coast and total cumulative upwelling anomalies for the Southern Benguela (r = −0.67, p < .001).

Re-evaluation of the environmental dependence of Pacific sardine recruitment

The environment influences the recruitment of small pelagic fishes, so environmental indices are used to match fishing mortalities to the stocks’ productivities. For example, the exploitation fraction for the northern stock of Pacific sardine in the Northeastern Pacific is a function of sea-surface temperature (SST). The functional relationship changes, however, because our perception of the environmental effects on sardine recruitment is based on assessment models that are periodically updated with new input data and assumptions. In this paper, we use data from recent stock assessments to re-examine previously identified correlations of sardine recruitment success (the logarithmic ratio of recruitment and spawning stock biomass) with indices of SST off Southern California and the Pacific Decadal Oscillation (PDO). We show that the earlier correlation with SST is likely invalid, and the persistent correlation with the PDO is weaker. Because many environmental stock-recruitment relationships fail upon re-examination, environmental proxies for fish productivity might not always prescribe the correct amount of fishing mortality and should be avoided. Alternatively, for species assessed periodically, dynamic fishing mortalities could be based on measurements of recent stock productivity inferred directly from surveys, or from the results of analytical assessments based on those observations.

Physical-biological interactions in the ocean: case studies from the northern Canary Current Upwelling Ecosystem

Physical-biological interactions in the ocean: case studies from the northern Canary Current Upwelling Ecosystem

Linking sardine recruitment in coastal areas to ocean currents using surface drifters and HF radar: a case study in the Gulf of Manfredonia, Adriatic Sea

Abstract. Understanding the role of ocean currents in the recruitment of commercially and ecologically important fish is an important step toward developing sustainable resource management guidelines. To this end, we attempt to elucidate the role of surface ocean transport in supplying recruits of European sardine (Sardina pilchardus) to the Gulf of Manfredonia, a known recruitment area in the Adriatic Sea. Sardine early life history stages (ELHSs) were collected during two cruises to provide observational estimates of their age–size relationship and their passive pelagic larval duration (PPLD). We combine these PPLDs with observations of surface ocean currents to test two hypotheses: (1) ELHSs are transported from remote spawning areas (SAs) by ocean currents to the Gulf of Manfredonia; (2) sardines spawn locally and ELHSs are retained by eddies. A historical surface drifter database is used to test hypothesis 1. Hypothesis 2 is tested by estimating residence times in the Gulf of Manfredonia using surface drifters and virtual particles trajectories that were computed from high-resolution observations of surface currents measured by a high-frequency (HF) radar network. Transport to the Gulf of Manfredonia from remote SAs seems more likely than local spawning and retention given a mismatch between observed PPLDs of 30–50 days and relatively short (<10-day) average residence times. The number and strength of connections between the gulf and remote SAs exhibit a strong dependence on PPLD. For PPLDs of 20 days or less, the gulf is connected to SAs on the western Adriatic coast through transport in the Western Adriatic Current (WAC). SAs on the east coast are more important at longer PPLDs. SAs in the northern and central Adriatic exhibit weak connections at all PPLD ranges considered. These results agree with otolith microstructure analysis, suggesting that the arrival of larvae in the gulf is characterized by repeated pulses from remote SAs. This is the first attempt to describe the processes related to Lagrangian connection to, and retention in, the Gulf of Manfredonia that will be complemented in the future using validated numerical ocean models and biophysical models.

Study of sardine (Sardina pilchardus) regime shifts in the Iberian Atlantic shelf waters

AbstractSardine fisheries in the Iberian Atlantic shelf (36°N–44.5°N) show decadal‐scale cycles. In the late 1990s, a positive phase in sardine stock was expected; on the contrary, catches have declined until now. Regime shifts in climatic and oceanographic variables on different scales (as forcing factor) and shifts in sardine stock (as result) have been used with the aim of identifying the physical variables that explain most of the sardine population variance in the region. Circa 1998, when last sardine regime shift was detected, the main patterns of large‐scale atmospheric circulation in the Northern Hemisphere with influence in the study area namely Northern Atlantic Oscillation (NAO) and East Atlantic (EA) pattern changed and coupled in a combination that led to a rise in sea surface temperature and a decline in the coastal upwelling intensity. Several years with a downwelling situation in average in the main spawning and feeding Iberian sardine areas would have affected the stock abundance, averting the return to the projected positive regime. The sardine negative regime shift was detected first in the regions of the study located further north. The regional variable latent heat flux that groups a set of environmental processes related to the ocean–atmosphere heat exchanges and so with the turbulence manages to explain the 72% of sardine recruitment.

Relationship between recruitment of Japanese sardine (Sardinops melanostictus) and environment of larval habitat in the low‐stock period (1995–2010)

AbstractStock level of Japanese sardine (Sardinops melanostictus) was high from 1980s to early 1990s and low from late 1990s to 2000s. The warm and cold water masses in the vicinity of the Kuroshio axis from winter to early spring used to be critical for the recruitment in the high‐stock period, because most of the larvae were distributed there. However, the environmental fluctuation might not affect the recruitment in the low‐stock period. Some studies reported that spawning location and spawning season, and hence the larval habitat, differ depending on the stock level. Three points were investigated in this study: (a) how spawning location and spawning season shifted from the late 1990s, (b) confirmation of the distribution area of larvae in the recent low‐stock period and (c) whether the water temperature in the vicinity of the Kuroshio axis was still related to the recruitment in the low‐stock period. The spawning location and spawning season clearly changed after 1995. Consequently, particle tracking experiments suggested that the larvae appeared in the vicinity of the Kuroshio axis from winter to early spring decreased. Nevertheless, only the ambient temperature of larvae that appeared in the vicinity of the Kuroshio axis from winter had a significant negative correlation with an index of the recruitment in the low‐stock period. It is suggested that the warm and cold masses in the vicinity of the Kuroshio axis are critical for the recruitment regardless of the stock level.

Climate variability and sardine recruitment in the California Current: A mechanistic analysis of an ecosystem model

AbstractRecruitment varies substantially in small pelagic fish populations. Understanding of the mechanisms linking environment to recruitment is essential for the effective management of fisheries resources. In this study, we used a fully coupled end‐to‐end ecosystem model to study the effect of climate variability on sardine recruitment in the California Current System during 1965–2006. Ocean variability was represented by ROMS hydrodynamic and NEMURO biogeochemical models, and sardine population dynamics was simulated through a full life cycle individual‐based model. Model analysis was designed to elucidate how changes in abiotic and biotic conditions may impact the spawning habitats, early life stage survival, and ultimately recruitment of sardine. Our findings revealed the importance of spatial processes to shape early life stages dynamics. Shifts in spawning habitats were dictated by the spatial variations in temperature and the behavioral movement of adults. Additionally, the spatial match of eggs with warmer temperatures and larvae with their prey influenced their survival. The northward shifts in spawning locations and the accomplishment of good recruitment in warmer years agreed with existing knowledge. Egg production and survival during egg and yolk‐sac larval stages were key factors to drive the long‐term variations in recruitment. Finally, our analysis provided a quantitative assessment of climate impact on year‐to‐year variation in sardine recruitment by integrating multiple hypotheses.

Improvement in recruitment of Japanese sardine with delays of the spring phytoplankton bloom in the Sea of Japan

AbstractTo evaluate the impact of temporal variation of primary productivity on the recruitment of Japanese sardine (Sardinops melanostictus) in the Sea of Japan, the phenology of sea surface phytoplankton abundance was estimated from 8 day multiple satellite (SeaWiFS, MODIS‐Aqua, MERIS, and VIIRS) derived sea surface chlorophyll (SSChl) a concentrations from January 1998 to December 2015. Because relationships between SSChl a and in situ chlorophyll a concentrations were significantly different among periods based on the satellite combinations used, maximum and minimum SSChl a concentrations of 1 year were relativized as 1 and 0, respectively. Spatio‐temporal variation of relativized SSChl a concentrations was determined by using empirical orthogonal function (EOF) analysis. Scores in the first EOF mode denoted the basin‐scale variations of SSChl a concentrations in the Sea of Japan, and the major peak from the end of February to the end of May displayed the spring bloom. The logarithm of recruitment per spawner (LNRPS) for sardine was positively affected by delays in the start and end dates of the spring phytoplankton bloom. The delay of the date of the lowest sea surface temperature contributed to the delay of the end of the spring bloom during the period 1998–2015 and elevated the LNRPS during the period 1982–2015. Sardine spawns in the southern Sea of Japan from April to May, hence, delays of the spring bloom prolonged its overlap with sardine larval periods, and thus improved the recruitment of Japanese sardine in the Sea of Japan.

Temperature and food-mediated variability of European Atlantic sardine recruitment

The influence of the environmental conditions during larval development on the resulting recruitment strength was investigated for European sardine (Sardina pilchardus) at Atlanto-Iberian waters. Satellite-derived Sea Surface Temperature (SST) and Chlorophyll-a concentration (Chla) data from the previous spawning seasons (January to March/April and October to December of the previous year) were related to recruitment success data in the main recruitment hotspots. Recruitment data was taken from yearly acoustic scientific cruises and from the ICES recruitment index estimated by an age-structured model for the entire stock. A linear discriminant analysis model using SST, Chla, and the abundance of spawners during the spawning season identified years of high and low recruitment for all the recruitment hotspots with an accuracy of ≥79%. In general, high recruitment years were associated with high Chla and low SST, although the most important variables to discriminate between the groups were area-specific. High recruitment years were mostly related to high food availability (Chla), particularly during the last quarter of the previous year. In Western Iberia and in the Gulf of Cadiz, high recruitment years were also associated to lower SST, whereas in the Bay of Biscay, where SST during the winter was generally below the optimal range ≈11–12 °C for sardine larval development, higher recruitment was associated with high SST. For ICES data of the southern European sardine stock, lower SST and higher Chla during the last quarter of the previous year were associated with high recruitment years and SST alone was able to discriminate between the two recruitment groups with 73% accuracy. Although the time-series of available data are still small, these significant relationships are consistent with field and laboratory studies relating larval growth and mortality with main environmental drivers. These relationships should be further investigated in the following years to evaluate if they can be used to construct reliable indicators to predict the level of recruitment and abundance with sufficient advance to help in the management of this important fishing resource.

A real-time PCR approach to detect predation on anchovy and sardine early life stages

Recruitment of sardine (Sardina pilchardus Walbaum, 1792) and anchovy (Engraulis encrasicolus Linnaeus, 1758) is thought to be regulated by predation of their eggs and larvae. Predators of sardine and anchovy can be identified by visual taxonomic identification of stomach contents, but this method is time consuming, tedious and may underestimate predation, especially in small predators such as fish larvae. Alternatively, genetic tools may offer a more cost-effective and accurate alternative. Here, we have developed a multiplex real-time polymerase chain reaction (RT-PCR) assay based on TaqMan probes to simultaneously detect sardine and anchovy remains in gut contents of potential predators. The assay combines previously described and newly generated species-specific primers and probes for anchovy and sardine detection respectively, and allows the detection of 0,001ng of target DNA (which corresponds to about one hundredth of the total DNA present in a single egg). We applied the method to candidate anchovy and sardine egg predators in the Bay of Biscay, Atlantic Mackerel (Scomber scombrus) larvae. Egg predation observed was limited primarily to those stations where sardine and/or anchovy eggs were present. Our developed assay offers a suitable tool to understand the effects of predation on the survival of anchovy and sardine early life stages.

Long-term variations in Ibero-Atlantic sardine (Sardina pilchardus) population dynamics: Relation to environmental conditions and exploitation history

In the current work we modeled the relationship between global atmospheric indices, seasonal sea surface temperature and coastal upwelling on the recruitment of the Ibero-Atlantic sardine. To study long-term Ibero-Atlantic sardine population dynamics a historical recruitment index (HRI) was developed from 1947 to 2012, for the Portuguese west coast, the main recruitment area of the Ibero-Atlantic sardine stock. HRI was calculated as the number of individuals ≤16.0cm total length per landed kg using length and weight distribution data from purse-seine fleet landings. Recruitment periodicity changed after the late 1960s from 2–3 years to 4 years cycles. Atlantic Multidecadal Oscillation (AMO), summer North Atlantic Oscillation (NAO), summer sea surface temperatures (SST) and winter upwelling were significantly related with recruitment, explaining 45.9% of recruitment variability in the generalized additive model. Possible mechanisms leading to those relationships were discussed. A complex two-way relationship between the sardine exploitation and recruitment appears to exist. High recruitments allowed the increase in total landings in the following years but high exploitation periods may have also led to a decrease of the population potential productivity in the subsequent years.

Effect of mesoscale eddies and streamers on sardine spawning habitat and recruitment success off Southern and central California

AbstractWe quantified the effect of mesoscale eddies and streamers on the spatial distribution of Pacific sardine spawning habitat using a merged altimetry data set and a statistical spawning habitat model. The distribution of eggs could be predicted using sea‐surface temperature, chlorophyll concentration, and eddy kinetic energy (EKE) similarly to previous studies. Eddies alone did not have a significant additional or emergent effect on the probability of capturing eggs beyond these predictors. Rather, mesoscale features (eddies and streamers) entrained water with the appropriate conditions in terms of temperature, chlorophyll, and EKE. These dynamic features moved appropriate spawning habitat for sardine offshore to areas where appropriate habitat otherwise would not exist. Using centroids of predicted sardine habitat, we showed that sardine recruitment success was inversely correlated with distance from shore of predicted sardine habitat centroids. This indicates that offshore transport has a negative effect on sardine recruitment, despite expanding favorable spawning habitat further offshore.

Rapid changes in growth, condition, size and age of small pelagic fish in the Mediterranean

Since 2007, the ecosystem of the Gulf of Lions has shifted to a different regime, characterised by a low anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) biomass and a remarkably high sprat (Sprattus sprattus) biomass. Surprisingly, the abundance and recruitment of anchovy and sardine remained high. To understand which processes (bottom-up or top-down control, etc.) could have caused this shift, we studied the changes in body condition, growth and size and age of anchovy, sardine and sprat over 1984–1985 and 1992–2012, using data from scientific surveys. The annual age structure of anchovy and sardine was estimated using Bayesian mixture models based on size frequency data with priors on the age–length relationship derived from independent otolith readings. The results indicated periods during which anchovy and sardine were in an average (1992–2004), good (2005–2007) or poor (2008–2012) overall state of condition. For sardine, the shift towards smaller fish observed during these past 4 years was explained by a combination of slower growth and the disappearance of older individuals (ages 2+). Despite the increase in biomass of sprat since 2008, indications were found that sprat was also smaller than in the past. As growth and condition decreased and overexploitation has not been documented or suspected for those three species in this area, we propose that the current decline in sardine and anchovy biomass could be due to qualitative and/or quantitative modifications in the planktonic production (i.e. a bottom-up control) or mass mortalities of adults due to an epidemic disease.

Environmental and parental control of Pacific sardine (Sardinops sagax) recruitment†

Abstract We confirm that sardine recruitment in the California Current, during the last three decades, mimics aspects of the environment in the North Pacific indicated by the Pacific Decadal Oscillation (PDO) index. The periods of stock increase and decrease followed consecutive years with positive and negative PDO values, respectively. During the “warm” periods, the average number of recruits per biomass was more than threefold higher than that during the “cold” periods. In addition to the environmental conditions experienced by the sardine larvae, we show that the variability in sardine recruitment is partially explained by the environmental conditions many months before the spawning season and the adult condition factor. We hypothesize that sardine have a metabolic deficit during spawning, so prior good feeding opportunities are necessary to increase both total fecundity and offspring robustness, to enhance both reproduction and survival, respectively. Our findings augment a century-old theory that the reproductive success of small pelagic fish is governed by the survival of the early life stages. The condition of each parent also matters. To predict sardine recruitment, we propose a “dual-phase” model based on seasonal PDO-based indices and a condition factor. The model identifies summer feeding seasons conducive to a good adult condition factor followed by spring-spawning seasons supportive of good larval retention and growth.