Sardinops Research Articles

Characterization of eight types of 17β-hydroxysteroid dehydrogenases from the Japanese sardine Sardinops melanostictus: The probable role of type 12a in ovarian estradiol synthesis

17β-hydroxysteroid dehydrogenases (Hsd17bs) play a critical role in sex steroid biosynthesis. Although multiple types of Hsd17b have been found in fish, there is limited research on their expression and function. Recently, we succeeded in identifying eight types of Hsd17b (types 3, 4, 7, 8, 10, 12a, 12b, and 14) by RNA sequencing in the Japanese sardine Sardinops melanostictus, a commercially important clupeoid fish; however, a homologous sequence of Hsd17b1, which catalyzes the key reaction of estradiol-17β (E2) synthesis, was absent. Here, we aimed to identify the Hsd17b type that plays a major role in E2 synthesis during ovarian development in Japanese sardine. The cDNAs encoding those eight types of Hsd17b were cloned and sequenced. The expressions of hsd17b3, hsd17b12a, and hsd17b12b were higher in ovary than in testis. In particular, hsd17b12a was predominantly expressed in the ovary. Expression of hsd17b3, hsd17b4, hsd17b12a, and hsd17b12b in the ovary increased during ovarian development. The enzymatic activities of Hsd17b3, Hsd17b12a, and Hsd17b12b were evaluated by expressing their recombinants in human embryonic kidney 293T cells. Hsd17b12a and Hsd17b12b catalyzed the conversion of androstenedione (AD) to testosterone (T) and estrone (E1) to E2. The results of in vitro bioassays using sardine ovaries indicated that E2 is synthesized from pregnenolone via AD and T, but not E1. These results suggest that Hsd17b12a plays a major role in E2 synthesis in sardine ovary by catalyzing the conversion of AD to T.

Investigation of inter-annual variation in the feeding habits of Japanese sardine (Sardinops melanostictus) and mackerels (Scomber spp.) in the Western North Pacific based on bulk and amino acid stable isotopes

Investigation of inter-annual variation in the feeding habits of Japanese sardine (Sardinops melanostictus) and mackerels (Scomber spp.) in the Western North Pacific based on bulk and amino acid stable isotopes

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.

Geographic, seasonal and ontogenetic variations of δ15N and δ13C of Japanese sardine explained by baseline variations and diverse fish movements

Understanding and predicting variability in the stable isotope ratios of nitrogen and carbon (δ15N and δ13C, respectively) of small pelagic fish is crucial to enable isotopic studies of a variety of marine predators that feed on them. However, because the isotope ratios reflect plastic feeding habits and fish migration in addition to baseline variation, their predictions require a mechanistic understanding of how each factor contributes. Here, we investigated the habitat-wide variability of δ15N and δ13C of the Japanese sardine Sardinops melanostictus in the western North Pacific and its marginal seas (the East China Sea and the Sea of Japan). By combining this with the archived particulate organic matter (POM) dataset as a baseline, we aimed to understand how ecological processes and baseline fluctuations affect isotope ratios of the sardine. Both δ15N and δ13C of sardine showed significant geographical and seasonal trends, with higher values in southern nearshore areas, including the Seto Inland Sea, intermediate values in marginal seas and lower values in Pacific offshore areas. As the variations were largely consistent with the geographic trend and the temporally integrated seasonal trend of isotope ratios of POM, respectively, the baseline variations are the main determinant of sardine isotope composition. The trophic positions of sardine are therefore not significantly different between regions, with possible minor increases in the southern nearshore area. Adults showed less geographic variation than larvae and juveniles, likely due to slower turnover periods and wider migration ranges. Although larval and juvenile isotope ratios in marginal seas mostly reflected the local baseline, those in the Pacific offshore often reflected the baseline in the neighbouring southern region, suggesting contrasting juvenile movements between regions. Our results suggest that the δ15N and δ13C of Japanese sardine strongly reflect baseline variations, but can also be influenced by life-stage- and region-dependent fish movements, thereby demonstrating both the possibility and difficulty of mechanistically modelling the isoscapes of lower trophic position species.

Catch as catch can: markets, availability, and fishery closures drive distinct responses among the U.S. West Coast coastal pelagic species fleet segments

Fishers often target multiple species. More diverse harvest portfolios may reduce income risk, increasing resilience to climate-driven changes in target species’ spatial distributions and availability. Moreover, different effects can be observed across vessels in response to the same shocks and stressors, as fishers are heterogeneous. Evaluation of climate risk across different vessel groups within a particular fishery requires consideration of heterogeneous climate impacts on the availability of multiple target species and how such changes may impact substitution behavior. Here we analyze how historical climate-driven changes in forage species distribution and the closure of the Pacific sardine fishery affected landings per vessel of three coastal pelagic species (CPS): Pacific sardine ( Sardinops sagax), market squid ( Doryteuthis opalescens), and northern anchovy ( Engraulis mordax) targeted by the U.S. West Coast CPS fleet from 2000 to 2020. Using cluster analysis, we grouped vessels into different fleet segments and estimated heterogeneous responses by fleet segment and port area. Our results show that considering heterogeneity is essential in the development of equitable and effective adaptation policies designed to mitigate the impact of changes on species availability in these fisheries.

Low hypoxia tolerance in larvae of the sardine Sardinops sagax and anchovy Engraulis encrasicolus may limit their stock recovery in the northern Benguela upwelling system

Physiological traits of five key fish species (Cape hake Merluccius spp., pelagic goby Sufflogobius bibarbatus, Cape horse mackerel Trachurus capensis, European anchovy Engraulis encrasicolus and sardine Sardinops sagax) from the northern Benguela upwelling system (NBUS) were compared during their larval stage by measurements of enzymatic activities of key metabolic enzymes (citrate synthase [CS] and pyruvate kinase [PK]). Two distinct age classes (early larvae: 8‒14 days old; late larvae: 22‒29 days old) for each species and from two areas were compared: Terrace Bay (20° S), the main spawning ground of Cape horse mackerel and anchovy, and Kunene (17° S), near the Angola–Benguela frontal zone, where warm and hypoxic water masses intrude into the NBUS. The results revealed significantly higher CS activity in both larval age classes in Cape horse mackerel, anchovy and sardine compared with Cape hake and pelagic goby. Pelagic goby and Cape horse mackerel had significantly higher PK activity compared with Cape hake, anchovy and sardine, apparent in both larval age classes and both areas. Results for anaerobic metabolism indicate higher capacity for pelagic goby and Cape horse mackerel to recover from oxygen debt built up in prey capture and predator escape behaviour and a higher potential for hypoxia tolerance when compared with Cape hake, anchovy and sardine. These results suggest higher survival probability for pelagic goby and Cape horse mackerel compared with the other species under conditions of a shoaling oxygen minimum zone and may explain their current dominance in the NBUS.

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.

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.

Impacts of regime shift on the fishery ecosystem in the coastal area of Kyoto prefecture, Sea of Japan, assessed using the Ecopath model

The present study assessed the impact of regime shifts on the ecosystems that support fisheries in the coastal area of Kyoto Prefecture, the Sea of Japan. Ecopath models were constructed during cold (1985) and warm (2013) regime periods, when the fishery structures were different. Both models evaluated by pre-balance diagnostics were reliably constructed. Multiple ecosystem indicator values were compared between the two periods to assess changes in the ecosystem structure and the impact of fisheries on the ecosystem. The major change in catches in the coastal area of Kyoto Prefecture between 1985 and 2013 was that the Japanese sardine drastically declined and Spanish mackerel increased. The primary production requirements of fisheries (%PPR) were higher in 1985, reflecting the greater impact of sardine. The mean trophic level of catch and system omnivory index suggested that stability of trophic network increased after the sharp decline in sardine biomass. The %PPR, the loss of secondary production due to fisheries exploitation and probability analysis of fishery sustainability suggested that the fishery structure in 2013, which was dominated by set nets, contributed to a more sustainable fishery than the structure in 1985, when fishery was dominated by purse seines and set nets.

Environmental DNA in the Kuroshio reveals environment-dependent distribution of economically important small pelagic fish

IntroductionSmall pelagic fishes constitute large proportions of fisheries and are important components linking lower and higher trophic levels in marine ecosystems. Many small pelagic fishes in the Northwest Pacific spawn upstream in the Kuroshio and spend their juvenile stage in the Kuroshio Front area, indicating that the Kuroshio Current system impacts their stock fluctuations. However, the distribution of these fish relative to the Kuroshio has not been determined due to dynamic spatio-temporal fluctuations of the system. Here, the recent development of environmental DNA (eDNA) monitoring enabled us to investigate the distribution patterns of four economically important small pelagic fishes (Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus, chub mackerel Scomber japonicus, and blue mackerel Scomber australasicus) in the Kuroshio Current system. MethodsThe influence of environmental factors, such as sea water temperature, salinity, oxygen concentration, chlorophyll-a concentration, and prey fish on the occurrence and quantity of target fish eDNA was analyzed using generalized additive models. In addition, the detection (presence) of target fish eDNA were compared between the offshore and inshore side areas of the Kuroshio axis.ResultsSea water temperature showed important effect, especially on the distribution of Japanese sardine and Japanese anchovy, whereas the distribution pattern of chub mackerel and blue mackerel was greatly influenced by the eDNA quantity of Japanese sardine and Japanese anchovy (especially potential prey fish: Japanese anchovy). In addition, we found that the four target fish species could be observed in areas on the inshore side or around the Kuroshio axis, while they were hardly found on the offshore side.ConclusionBased on eDNA data, we succeeded in revealing detailed spatial distribution patterns of small pelagic fishes in the Kuroshio Current system and hypothesized predator–prey relationships influence their distribution in small pelagic fish communities.

Metabarcoding Used for the First Time to Identify Prey of Wild Totoaba macdonaldi.

Totoaba macdonaldi is an endangered endemic fish of the Gulf of California. Overexploitation resulted in the Mexican government banning the fishing of this species in 1975, and it being listed as endangered. However, the species is still subject to illegal fishing. Despite its conservation status, little is known about totoaba biology. The present study aimed to implement, for the first time, a metabarcoding protocol to describe the totoaba diet. Four wild totoaba individuals, seized by Mexican law enforcement agents, were dissected, and their stomach contents were collected. Three representative amplicon libraries were generated for cephalopods, chordates, and eukaryotes. After sequencing, 18 different taxa were identified, of which 11 species were recognized as prey. The totoaba were found to have consumed Pacific anchovy (Cetengraulis mysticetus), flathead grey mullet (Mugil cephalus), bigeye croaker (Micropogonias megalops), northern anchovy (Engraulis mordax), ocean whitefish (Caulolatilus princeps), milkfish (Chanos chanos), and Pacific sardine (Sardinops sagax). Members of the Euphausiidae family (krill) were also identified. This study identified up to four times more species in much fewer samples than previous studies based on morphological recognition, thus confirming metabarcoding as an effective method for studying the feeding habits of this species and one providing the tools required for further analysis of the totoaba diet.

An integrated catch-at-age model for analyzing the variability in biomass of Pacific sardine (Sardinops sagax) from the Gulf of California, Mexico

Knowledge of the population dynamics and changes in the abundance of Pacific sardine fishery is relevant for fishery management. In this study, an integrated catch-at-age model (ASAP) was performed on 47 fishing seasons (1971−2018) ofSardinops sagaxin the Gulf of California, including five indices of relative abundance (survey catch rate, acoustic surveys, egg and larvae, spawning probability, and bird diet index). The ASAP model showed the importance of age group 0 (recruits); this is the most important age group in the Pacific sardine fishery, and its abundance has consequences for the variability of the total abundance of the species. In this study, the spawning and acoustic indices denoted the changes in abundance of Pacific sardine population better than the survey catch rate and bird diet ratio. Historically, the total biomass is estimated to have varied from 461 thousand to 4.82 million t. In the 1970s and 1980s, a peak biomass close to three million tons was reached, and in the second peak from the mid-1990s to the late 2000s, the total biomass varied between 3.26 and 4.82 million t. When the spawning biomass was at least 750 x 103t, the recruitment (age group 0) showed high values, promoting high catches in the subsequent years. The annual fishing mortality estimates did not show a rise related to the periods of high harvestable biomass; therefore, the catch records were low in comparison to the harvestable biomass estimated from ASAP, and the annual fishing mortalities were less than 0.22 during periods of high abundance. The changes in biomass and the variability observed in the Pacific sardine fishery were validated for the model diagnostics; the retrospective patterns did not identify poor fits to data, therefore the robustness of the model can be used for management purposes. According to the Kobe phase plot, the Pacific sardine fishery in the Gulf of California has been fully exploited with an apparent stable level of harvest.

Assessment and management of the temperate stock of Pacific sardine (Sardinops sagax) in the south of California Current System

Knowing the population dynamics and the variations in the abundance of the Pacific sardine is essential for suitable fishing management. Due to the possibility that the different stocks of Sardinops sagax that can inhabit the south of California Current System (CCS) were present in the catches, the caught of the temperate stock were discriminated from the total landed and, subsequently was assessed the dynamics of temperate stock of Pacific sardine (TSPS) for the period from 1989 to 2021. For the above, we use a statistical analysis of catch-at-age (ACE), which is an integrated analysis model that includes three indices of relative abundance (catch rate, acoustic surveys and evaluations of eggs and larvae). Acoustic indices and capture rate (CPUE) denoted changes in population abundance better than the index of eggs and larvae. The total biomass has shown great interannual variability oscillating between 853,476 and 1,592,519 t; a similar trend was shown by the spawning biomass, oscillating between 404,189 t and 770,484 t. With the modified Ricker Stock-Recruitment model, the minimum biomass Bmin = 50,000 t and the exploitation rate in Emrsy = 0.251 year−1 were estimated. Considering behavior of proportion between the recorded catch and the estimated biologically acceptable catch to each year (Cobs/BAC), we can infer that the TSPS has been sustainably exploited throughout analyzed period, except for 2014 and 2017 fishing seasons, when the BAC was exceeded by around 20%. Harvest control rule, by depending on a biomass fraction, estimated with an integrated model ACE, is considered a management suitable strategy to TSPS.

Archived DNA reveals marine heatwave‐associated shifts in fish assemblages

AbstractMarine heatwaves can drive large‐scale shifts in marine ecosystems, but studying their impacts on whole species assemblages is difficult. Analysis combining microscopic observations with environmental DNA (eDNA) metabarcoding of the ethanol preservative of an ichthyoplankton biorepository spanning a 23 years time series captures major and sometimes unexpected changes to fish assemblages in the California Current Large Marine Ecosystem during and after the 2014–2016 Pacific Marine Heatwave. Joint modeling efforts reveal patterns of tropicalization with increases in southern, mesopelagic species and associated declines in commercially important temperate fish species (e.g., North Pacific Hake [Merluccius productus] and Pacific Sardine [Sardinops sagax]). Data show shifts in fisheries assemblages (e.g., Northern Anchovy, Engraulis mordax) even after the return to average water temperatures, corroborating ecosystem impacts found through multiple traditional surveys of this study area. Our innovative approach of metabarcoding preservative eDNA coupled with quantitative modeling leverages the taxonomic breadth and resolution of DNA sequences combined with microscopy‐derived ichthyoplankton identification to yield higher‐resolution, species‐specific quantitative abundance estimates. This work opens the door to economically reconstruct the historical dynamics of assemblages from modern and archived samples worldwide.

Assessment and management recommendations for the status of Japanese sardine Sardinops melanostictus population in the Northwest Pacific

Japanese sardine (Sardinops melanostictus) is one of the main species caught in the high seas purse seine fishery in the Northwest Pacific Ocean and its resources gradually recovered in recent years. This paper used the historical yearly catch data of Japanese sardine from 1990 to 2021 and biological data (manily for body length) in April-November 2020–2022 to assess its maximum sustainable yield (MSY) and stock resource status using Catch-MSY and LBB models, respectively. And then based on the evaluation results of Japanese sardine MSY, we analyzed the stock management strategies on this species by the Kobe II Strategy Matrix (K2SM). The results were as follows: (1) the Catch-MSY model assessed its MSY as 170.0 × 104 t with a 95 % confidence interval of 134.1 × 104 t ∼ 211.4 × 104 t; (2) the LBB model assessed the biological parameters for the last three years (2020–2022) with the interval range of Lc/Lc_opt was 0.76–0.95, B/BMSY was 0.79–0.84, and B/B0 was 1.9–2.3, which indicate that Japanese sardine resource is healthy and recovering; (3) the results of these two models and the Kobe Ⅱ management strategy matrix are combined to predict future resource status probability values. The management target range for the medium term (5-10 years) should be set 100 % TAC to MSY values, i.e., 102.0 × 104 t–170.0 × 104 t as TAC, and for the long term (10–20 years) can be set to 120 % TAC to MSY values, i.e., 122.4 × 104 t–170.0 × 104 t. Japanese sardine fishery resource can be sustainably used on a longer time scale.

Spatio-Temporal Variations in the Potential Habitat Distribution of Pacific Sardine (Sardinops sagax) in the Northwest Pacific Ocean

Pacific sardine (Sardinops sagax) is a commercially important species and supports important fisheries in the Northwest Pacific Ocean (NPO). Understanding the habitat distribution patterns of Pacific sardine is of great significance for fishing ground prediction and stock management. In this study, both single-algorithm and ensemble distribution models were established through the Biomod2 package for Pacific sardine by combining the species occurrence data, sea surface temperature (SST), sea surface height (SSH), sea surface salinity (SSS) and chlorophyll-a concentration (Chla) in the NPO during the main fishing season (June–November) from 2015 to 2020. The results indicated that the key environmental variables affecting the habitat distribution of Pacific sardine were the SSH and SST. The suitable habitat area for Pacific sardine showed significant monthly changes: the suitable habitat range in June was larger than that in July and August, while the suitable habitat range gradually increased from September to November. Furthermore, the monthly geometric centers of habitat suitability index (HSI) for Pacific sardine presented a counterclockwise pattern, gradually moving to the northeast from June, and then turning back to the southwest from August. Compared with single-algorithm models, the ensemble model had higher evaluation metric values and better spatial correspondence between habitat prediction and occurrence records data, which indicated that the ensemble model can provide more accurate prediction and is a promising tool for potential habitat forecasting and resource management.

A Quarter Century of Monitoring the Fish Assemblages of San Diego Bay, California from 1995 to 2019

A time series of 22 samples collected in April and July were taken of the fish assemblages of San Diego Bay over 11 non-consecutive years from 1995 to 2019. Each sample consisted of four ecoregions using a variety of collection methods designed to assess all components of the ichthyofauna. These samples yielded a total of 525,288 fishes belonging to 90 species and weighing 3,507 kg over the 25-year period. Northern anchovy was the most abundant fish species comprising 41% of the total catch despite its virtual absence near the end of the survey period, followed by topsmelt, slough anchovy, shiner perch, and Pacific sardine. Round stingrays dominated in weight constituting more than 27% of the total biomass taken followed by spotted sand bass, and northern anchovy. Approximately 64% of all individual fish captured in San Diego Bay during this study were juveniles. In a canonical correlation analysis, temperature, distance from the mouth of the bay, and salinity accounted for nearly 93% of the variance in individual species abundances. In the time series analyses, we found all three potential temporal patterns of fish species abundance, biomass, and diversity, namely: 1) significant decreases over time, 2) significant increases over time, and 3) no significant change over time. Abundance of eight of the top 35 species (including northern anchovy, topsmelt, slough anchovy, and shiner perch) and all forage species combined decreased over the study. Two species increased significantly in abundance, spotted sand bass and dwarf perch. Whereas, total abundance, total biomass, species richness, Shannon diversity, and the majority (71%) of species abundances did not change over the 25-year period. Despite various environmental perturbations and the general trends of decreases in larval and fish abundance indices over the Southern California Bight in recent years, the stability in species richness and composition over time reflects the generally resilient nature of the fish assemblage structure of San Diego Bay that has been maintained by active management including restoration practices.

Projecting climate change impacts from physics to fisheries: A view from three California Current fisheries

Motivated by a need for climate-informed living marine resource management, increased emphasis has been placed on regional end-to-end modeling frameworks designed to project climate impacts on marine ecosystems and evaluate the efficacy of potential management strategies under changing conditions. The ‘Future Seas’ project was initiated with a focus on three fisheries (Pacific sardine, swordfish, and albacore tuna) in the California Current System (CCS). This work leverages a suite of climate, ocean, ecosystem, and economic models to project physical, ecological, and socio-economic change, evaluate management strategies, and quantify uncertainty in model projections. Here we describe the components of the modeling framework, considerations underlying choices made in model development, engagement with stakeholders, and key physical, ecological, and socio-economic results to date, including projections to 2100. Our broad aims are to (i) synthesize a large body of climate and fisheries research that has been conducted, and continues, under the Future Seas umbrella, and (ii) provide insight and recommendations to those pursuing similar efforts for other applications and in other regions. In general, our results indicate that all three species will likely shift their distributions (predominantly poleward) in the future, which impacts accessibility to fishing fleets, spatial management, and quota allocation. For similar integrative climate-to-fisheries projections, we recommend attention is given to: recognizing potential biases arising from differences between the climate products used for ecological model fitting and those used for model projection; how sources of projection uncertainty are prioritized, incorporated, and communicated; and quantitatively linking scenarios – especially socio-economic scenarios – with climate and ecological projections.

Biological interactions potentially alter the large-scale distribution pattern of the small pelagic fish, Pacific saury Cololabis saira

We aimed to understand the effect of biological interactions on the distribution of small pelagic fishes. Surveys were conducted during 2003-2019 using sea surface trawl nets in the western and central North Pacific Ocean, covering an area between 143° E and 165° W, to estimate the distributions of 4 small pelagic fishes: Pacific saury Cololabis saira, Japanese sardine Sardinops melanostictus, chub mackerel Scomber japonicus, and Japanese anchovy Engraulis japonicus. These species are potential competitors, as they all occupy a similar niche. Japanese anchovy was abundant during 2003-2012, while Japanese sardine and chub mackerel were abundant during 2013-2019. Only Pacific saury was distributed throughout the survey area, while the others occurred within the area west of 180° longitude (WA). Pacific saury and its potential competitors showed adjacent distributions, with a slight overlap along sea surface temperature gradients in the WA. This distribution pattern remained even when Japanese sardine expanded its distribution to colder waters and increased in abundance after 2013, resulting in a shift in the distribution of Pacific saury to colder waters and a period of high densities of Japanese sardine. Such a distribution shift of Pacific saury was not observed in the area east of 180° longitude, where no Japanese sardine was observed. These results suggest the possibility that biological interactions such as inter-species competition can be an important factor in determining the distribution of small pelagic fishes.

Feeding ecology of coexisting Heermann’s gull (Larus heermanni) and elegant tern (Thalasseus elegans) chicks, based on stable isotope measurements

The Heermann’s gull and the elegant tern are considered Near Threatened by the IUCN Red List of Threatened Species and under special protection under Mexican environmental law. Close to 95% of the world populations of both species breed on Isla Rasa (Gulf of California). As both species breed on the same island, competition for trophic resources can be expected to be high. Here, the feeding ecology of chicks of both species, as well as the contribution by prey species provided by adults, was studied using stable carbon (δ13C) and nitrogen (δ15N) isotope analysis in 3 consecutive breeding seasons (2017, 2018, and 2019). Results of the stable isotope analysis revealed that although some segregation in the δ13C and δ15N values exists between both species, isotopic mixing model outputs indicated that fishes were the most important prey group for both. However, at the prey species level, the most important prey consumed by Heermann’s gulls were northern anchovy, Pacific sardine, and Pacific mackerel, followed by pelagic crab, whereas elegant terns consumed mainly northern anchovy. The trophic width estimated with the stable isotope values during 2017 and 2019 was higher for the Heermann’s gull than for the elegant tern. In contrast, for 2018, the trophic width of the elegant tern was higher than that of the Heermann’s gull. In addition, both species showed a high degree of trophic width overlap during 2017 and 2019 (overlap probability 0.67 and 0.79, respectively). These results highlight the ability of the Heermann’s gull to redirect its food intake to what might be the most accessible prey groups, a feeding strategy that allows the coexistence.