Distribution Of Marine Species Research Articles

Seasonal variation in marine bird distribution in the northeast Pacific Ocean

Human activities have profound influences on marine ecosystems. Marine birds are particularly sensitive to these impacts and, given their ease of observation and diverse life histories, often represent good indicators of ecosystem health. Conserving marine birds and their ecosystems requires robust predictions of species distribution to help mitigate human disturbance in areas where large aggregations of diverse species occur. We modelled variation in marine bird species diversity (Shannon-Wiener Index) and taxonomic family level probability of occurrence to map the intensity and extent of highly diverse ‘hotspots’ in Canada’s Pacific Exclusive Economic Zone. To do so, we paired 20 yr of survey data from the North Pacific Pelagic Seabird Database (1997-2017) and remote sensing data describing marine conditions and local geography (sea surface temperature, chlorophyll a, bathymetry, distance to shore, and benthic substrate type). These data were used to illustrate how seasonality within years and the El Niño-Southern Oscillation (ENSO) across years influenced spatial patterns in diversity. Hotspots were most persistent in Hecate Strait, off the west coast of Vancouver Island, and surrounding the Scott Islands in most seasons. Changes in hotspot locations and intensity were observed across seasons and within season under varying ENSO conditions. Our results provide a template for mapping marine species distribution for the purpose of identifying hotspots of diversity, and thereby facilitate planning to minimize harmful impacts in highly diverse and dynamic systems.

Environmental DNA recovers fish composition turnover of the coral reefs of West Indian Ocean islands.

Islands have been used as model systems to study ecological and evolutionary processes, and they provide an ideal set-up for validating new biodiversity monitoring methods. The application of environmental DNA metabarcoding for monitoring marine biodiversity requires an understanding of the spatial scale of the eDNA signal, which is best tested in island systems. Here, we investigated the variation in Actinopterygii and Elasmobranchii species composition recovered from eDNA metabarcoding along a gradient of distance-to-reef in four of the five French Scattered Islands in the Western Indian Ocean. We collected surface water samples at an increasing distance from reefs (0 m, 250 m, 500 m, 750 m). We used a metabarcoding protocol based on the 'teleo' primers to target marine reef fishes and classified taxa according to their habitat types (benthic or pelagic). We investigated the effect of distance-to-reef on β diversity variation using generalised linear mixed models and estimated species-specific distance-to-reef effects using a model-based approach for community data. Environmental DNA metabarcoding analyses recovered distinct fish species compositions across the four inventoried islands and variations along the distance-to-reef gradient. The analysis of β-diversity variation showed significant taxa turnover between the eDNA samples on and away from the reefs. In agreement with a spatially localised signal from eDNA, benthic species were distributed closer to the reef than pelagic ones. Our findings demonstrate that the combination of eDNA inventories and spatial modelling can provide insights into species habitat preferences related to distance-to-reef gradients at a small scale. As such, eDNA can not only recover large compositional differences among islands but also help understand habitat selection and distribution of marine species at a finer spatial scale.

Beta diversity and regionalization of the western Atlantic marine biota

AbstractAimUnderstanding the factors determining marine community variations is important for biogeography and conservation. Beta diversity is a metric for mapping species composition variations between communities and regionalizing biota. Ecoregions are commonly used for regionalization, but their empirical testing has been limited. Our aim is to map marine species composition variations in the Western Atlantic, identify variables related to these variations, and regionalize areas based on community distribution. Additionally, we test whether currently proposed ecoregions represent unique biota units and specific environmental conditions.LocationWestern Atlantic Ocean.TaxonVertebrates, invertebrates and algae.MethodsWe constructed a large marine biodiversity database, including vertebrates, invertebrates, and algae, totalling over 4 million records. We used the generalized dissimilarity model (GDM) to identify variables most related to species composition variations and map beta‐diversity variations. We employed an unsupervised classifier for community regionalization. To test if the ecoregion regionalization boundaries are corroborated by species distribution data, we used the Sørensen index. To assess if ecoregions correspond to environmental units, we checked if areas had distinct environmental conditions using a PCA of 134 marine environmental variables.ResultsThe GDM explained a high variation in species composition, 61% in the complete database. Analysing vertebrates, invertebrates and algae separately also yielded relatively high results: 46%, 54%, and 33%, respectively. Coastal areas differed from open sea areas in composition. Environmental variables combined better explained beta diversity than isolated variables. The regionalization based on GDM was not congruent with ecoregion boundaries. Moreover, ecoregions showed no distinction in species composition or environmental conditions.Main ConclusionsThis study's regionalization is crucial for marine biodiversity conservation, focusing on understanding species composition patterns between coastal and open sea areas to develop tailored conservation strategies. Despite sampling limitations, the study advances marine biogeography knowledge by analysing over 4 million species records and 134 environmental variables. This comprehensive approach enhances understanding marine species distribution and diversity and aiding the development of effective conservation measures.

Decadal‐Scale Variability in the Surface Flow of the Gulf of Maine Coastal Current: The Impact of Changing Climate Conditions on Coastal Circulation

AbstractThe Gulf of Maine (GOM) is one of the most productive and ecologically important marine environments on the planet. The Gulf of Maine Coastal Current (GMCC), which stretches along the western side of the GOM from the Bay of Fundy to Cape Cod, plays an important role in supporting this productivity by transporting nutrients, larvae and dinoflagellates southwestward along the Canadian and New England coastlines. Climate change has led to alterations in GOM ecosystems in recent years: GOM water temperatures have warmed and distributions of marine species have shifted. However, the impacts of the changing climate on the circulation within the GOM, including the path and velocity of the GMCC, have remained unclear. Here, we examine data collected by satellite tracked surface drifters and velocity data collected by the NERACOOS buoy array in order to investigate inter‐decadal changes to the flow of the GMCC. We find that over inter‐decadal timescales, the alongshore surface flow of the GMCC has slowed, particularly in the region southwest of Penobscot Bay. The slowdown observed in the GMCC surface flow is most likely attributed to a local strengthening of the southwesterly winds over the time period of interest.

Biological and environmental covariates of juvenile sockeye salmon distribution and abundance in the southeastern Bering Sea, 2002-2018.

Climate change is altering the distribution and abundance of marine species, especially in Arctic and sub-Arctic regions. In the eastern Bering Sea, home of the world's largest run of sockeye salmon (Oncorhynchus nerka), juvenile sockeye salmon abundance has increased and their migration path shifted north with warming, 2002-2018. The reasons for these changes are poorly understood. For these sockeye salmon, we quantify environmental and biological covariate effects within spatio-temporal species distribution models. Spatio-temporally, with respect to juvenile sockeye salmon densities: (1) sea surface temperature had a nonlinear effect, (2) large copepod, Calanus, a minor prey item, had no effect, (3) age-0 pollock (Gadus chalcogrammus), a major prey item during warm years, had a positive linear effect, and (4) juvenile pink salmon (O. gorbuscha) had a positive linear effect. Temporally, annual biomass of juvenile sockeye salmon was nonlinearly related to sea temperature and positively related to age-0 pollock and juvenile pink salmon abundance. Results indicate that sockeye salmon distributed with and increased in abundance with increases in prey, and reached a threshold for optimal temperatures in the eastern Bering Sea. Changes in population dynamics and distribution of sockeye salmon in response to environmental variability have potential implications for projecting specific future food securities and management of fisheries in Arctic waters.

Projections of climate-driven biogeographical changes of benthic mollusks in the Yellow Sea and East China Sea

Climate change influences the distribution of many marine species. To project the biogeographical changes of benthic mollusks in response to climate change in the Yellow Sea and East China Sea, ensemble species distribution models (SDMs) were applied. Ensemble SDMs performed well for ten of the thirteen selected benthic mollusks with environmental variables including temperature, salinity, current velocity, and depth. Six cold water mollusks, including bivalves Acila mirabilis, Ennucula niponica, Ennucula tenuis, Nuculana yokoyamai, Pendaloma otohimeae and Megayoldia japonica, were projected to contract their habitats and move northward in 2050s and 2100s under all of the RCP2.6, 4.5, 6.0 and 8.5 climate scenarios, with temperature being the most important environmental variable. Two warm water mollusks (bivalves Nucula tokyoensis and Leptomya minuta) were projected to lose their suitable habitats under future climate scenarios (all RCP scenarios), while two (the gastropod Cylichna biplicata and the bivalve Moerella hilaris) were projected to expand their habitats to the deeper water area. The most important environmental variable varied among warm water species between temperature, salinity and depth. This study will contribute to better understanding the marine species biogeographical changes under climate change, and thus we can better protect their biodiversity.

Spatial pattern of sea surface temperature and chlorophyll-a trends in relation to hydrodynamic processes in the Alborán Sea

Environmental conditions such as temperature, planktonic biomass, and ocean currents play an important role in the development and distribution of marine species. This work aims to estimate, in a high spatial resolution, the actual trends of sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) and to assess the relationship with the local hydrodynamic conditions in the Alborán Sea. To investigate these objectives, time series of SST and Chl-a of satellite sensor data were analyzed during 20 years from January 2001 to December 2020, using the Seasonal-Trend-Loess (STL) decomposition method and the Mann-Kendall seasonality test. The results, obtained with a 95% of confidence, showed that the Alborán Sea basin is subject to sea surface warming evaluated at 0.027 ± 0.008 ° C per year, related to the warming of the Atlantic water mass, which contributes to a decrease of productivity evaluated at -0.0024 ± 0.0003 μg /l per year of Chl-a concentration. These trends are not homogeneous over the entire basin area but show a large regional variation between different parts of the Alborán Sea due to the hydrodynamic process of the Atlantic Jet - Western Alboran Gyre system (AJ-WAG), which is more active especially in summer/autumn seasons and contribute largely to these changes by mixing the waters of the Atlantic Ocean and the Mediterranean Sea.

Sea turtle density surface models along the United States Atlantic coast

Spatially explicit estimates of marine species distribution and abundance are required to quantify potential impacts from human activities such as military training and testing, fisheries interactions, and offshore energy development. There are 4 protected species of sea turtle (loggerhead, green, Kemp’s ridley, and leatherback) commonly found along the east coast of the USA, our study area, and which require impact assessments. Data from 7 different survey organizations were used to create density surface models for the 4 sea turtle species utilizing 1.2 million km of line-transect surveys. A substantial portion (29.7%) of available sightings were not identified to the species level. Not including these sightings would underestimate density, so a conditional random forest model was used to assign unidentified sightings to species. Higher densities of loggerhead, green, and Kemp’s ridley sea turtles were predicted south of the Outer Banks in cool months, transitioning northwards in late spring to occupy seasonal neritic habitats. The highest leatherback densities were predicted off the coasts of Georgia and Florida. Leatherbacks were also predicted throughout offshore areas. The predicted distribution patterns generally matched satellite tracking and strandings data, indicating the models reproduced established seasonal movements. Surveys rarely detect sea turtles smaller than 40 cm, so these age classes are not represented. The models are the first for the study area to apply availability bias estimates developed in or near the study area and attempt to classify unidentified sightings to the species level, providing an updated, critical tool for conservation management along the eastern seaboard.

Transcriptional responses of Crassostrea hongkongensis under high and low salinity stress

Salinity, a key limiting factor, affects the distribution and survival of marine species. The Hong Kong oyster (Crassostrea hongkongensis), a euryhaline species found along the coast of the South China Sea, has become a major aquaculture bivalve species. To determine the molecular mechanism by which oysters respond to coastal waters with varying salinity levels, we used RNA-seq to sequence the gill samples of oysters exposed to normal (25 ‰, S25), low (5 ‰, S5) and high (35 ‰, S35) salinity conditions for one month. The results revealed different expression transcriptome levels among oysters living under low and high salinity conditions. Using high-throughput sequencing, we identified 811 up-regulated genes and 769 down-regulated genes. As determined by KEGG pathway mapping, the differentially expressed genes (DEGs) were significantly enriched in the prion diseases, histidine metabolism, arginine and proline metabolism, and beta-alanine metabolism pathways in both the S5 vs. S25 and S35 vs. S25 group comparison. Several DEGs including heat shock 70 kDa protein 12B-like, poly (ADP-ribose) polymerase (PARP), and tripartite motif-containing protein 2 (TRIM2), and low-density lipoprotein receptor-like, as well as KEGG pathways, including arginine and proline metabolism, apoptosis, PPAR signaling pathway, the thyroid hormone signaling pathway, were concerning response to salinity stress. Additionally, eight DEGs involved in salinity adaptation were selected for RT-qPCR validation, and the results confirmed the credibility of the transcriptome sequencing data. Overall, we designed a one-month, medium-term experiment to examine the responses of C. hongkongensis exposed to different levels of salinity stress and performed transcriptome analysis using high-throughput sequencing. Our results enhance current understanding of the molecular mechanisms of salinity stress responses in C. hongkongensis and provided insights into the osmotic biology of oysters.

Alaska's emergent fisheries processes

Under climate change, shifts in distributions of commercialized marine species challenge livelihoods and management in some fisheries and create opportunities in others. Whether existing management processes can enable fishers to access emergent fisheries – efforts by fishers to commercialize species – is unclear. Thus far, the literature has largely focused on reactive management processes that lead to overharvest and conflicts over fisheries allocations under species distributional shifts. We examine the progress and outcomes of 144 emergent fisheries in the state waters of Alaska, illustrating the historical diversity of management mechanisms, regions, species, and gears in the state's unique system of permitting and data collection for emergent fisheries. We further examine 28 emergent fisheries' roles in fishing portfolios through a métier analysis, finding that most are small extensions to existing portfolio strategies instead of novel opportunities for specialization. Together, these findings underscore challenges in adapting current processes to future shifts in marine species distributions, reflecting the need for large scale reconsiderations of scale, tradeoffs and a more holistic approach.

Disjunct habitat of cryptic Terebellides (Annelida, Trichobranchidae) species shows a phylogenetic link between polychaetes from the White and the North Seas

Understanding the distribution and biodiversity of marine species is crucial for developing effective conservation strategies and maintaining the health of global ecosystems. Advancements in molecular data utilization have significantly improved our understanding of biodiversity within the genus Terebellides. In this study, we conducted a phylogenetic analysis on polychaete samples from the Kandalaksha Bay, White Sea, revealing their affiliation with a putative undescribed species of the genus Terebellides found in two locations of the North Sea. Interestingly, this species was not detected in the Norwegian and Barents Seas, leading us to propose a disjunct distribution scenario for this Terebellides species. This unique distribution pattern might be attributed to the succession of polychaetes by new species, facilitated by the Gulf Stream and a climate change role in driving shifts in species' ranges and altering marine ecosystem dynamics.

Global trends of fronts and chlorophyll in a warming ocean

Ocean fronts affect phytoplankton and higher trophic levels, including commercially important fisheries. As the oceans warm, uncertainty remains around the trends in fronts. Here we examine changes in sea surface temperature fronts (frequency, density, and intensity) and the concentration of chlorophyll, over recent satellite records (2003 – 2020) in ocean warming hotspots - areas that are warming faster than other parts of the ocean. Commonalities exist across hotspots with comparable dynamics. Most equatorial and subtropical gyre hotspots experienced a decline in frontal activity (frequency, density, strength) and chlorophyll concentration, while in high-latitude hotspots, frontal activity and chlorophyll concentration mostly increased. Continued warming may accentuate the impacts, changing both total biomass and the distribution of marine species. Areas with changing fronts and phytoplankton also correspond to areas of important global fish catch, highlighting the potential societal significance of these changes in the context of climate change.

The climate crisis affects Mediterranean marine molluscs of conservation concern

AbstractAimThe Mediterranean Sea is a hotspot of endemism and it is warming at two to three times the rate of the global ocean. The continental masses that surround it constrain species range shifts. Therefore, as seawater temperature increases, distributions of marine species may contract into few small cooler refugia, with consequent risk of extinction from range contraction and fragmentation, particularly for endemics. We assess this risk for 15 molluscs of conservation concern occurring in the Mediterranean Sea. Invertebrates have been rarely assessed so far, despite their diversity and their importance for ecosystem functioning.LocationThe Mediterranean Sea and Eastern Atlantic Ocean.MethodsWe used ensemble species distribution models to project the future range of our target species and assess their IUCN Red List status.ResultsAs hypothesized, our models predicted major range shifts, contractions and fragmentation in the Mediterranean Sea for all species. Under moderate emission scenarios such as RCP4.5, up to 12 of the 15 target species will be eradicated from the eastern and southern warmer sectors. Under severe emission scenarios such as RCP8.5 and by the end of the century, we predict the global extinction of the endemic Patella ferruginea and Pinna nobilis. Non‐endemic species may buffer range loss in the Mediterranean with range gains in the Atlantic Ocean. Still, we predict that only a single species, Zonaria pyrum, will have marginal total range gains in future climate warming scenarios.Main conclusionsEight species will become threatened and the fate of Patella ferruginea and Pinna nobilis contrasts the belief that marine species are less prone to extinction than continental species under a warming climate.

MtDNA data reveal disparate population structures and High Arctic colonization patterns in three intertidal invertebrates with contrasting life history traits

IntroductionPost-glacial climate variation is known to have influenced the distribution of marine species in the North Atlantic. In particular, the Atlantic side of the Arctic has experienced strong fluctuations in both atmospheric and sea surface temperature, as well as seasonal ice coverage since the last glacial maximum (LGM). Here, we aim to unveil the phylogeography and historical demography of three rocky intertidal marine invertebrates showing a trans-Atlantic distribution and presently inhabiting the Arctic: Gammarus oceanicus, Littorina saxatilis and Semibalanus balanoides.MethodsWe used a large amount of mitochondrial DNA barcode data, both newly-obtained and stored in public databases. We performed phylogeographic and demographic analyses on 1119 G. oceanicus, 205 L. saxatilis, and 884 S. balanoides sequences.ResultsOur results show that all three of these boreal species have expanded their effective population sizes in the high Arctic Svalbard Archipelago since the LGM. Analyses investigating the origin of all these populations point to the eastern Atlantic.DiscussionBased on our results we conclude that the expansion of these boreal species to the Arctic possibly happened during an earlier warm cycle of the Holocene era, and is probably not the result of the recent ‘Atlantification’ of the Arctic. We also discuss the effects of dispersal potential on population structure as an important aspect of comparative biogeographical studies.

Using opportunistic data to study the distribution and abundance of a warm water elasmobranch at the northern edge of its range

Abstract Detecting changes in the distribution and abundance of marine species that are cryptic or occurring in very low abundances is difficult, but essential for assessing their status and informing management. One way of quantifying these changes is through the collation of opportunistic records. We reconstruct the population trajectory and distribution of the common stingray Dasyatis pastinaca around Great Britain, using opportunistic records, mostly obtained by recreational anglers. We tested if D. pastinaca declined in abundance and body size in response to fishing and if their distribution has shifted northwards in response to warming seas. We obtained 518 records covering the period 1838–2020. After correcting for observation effort, D. pastinaca catches reported by anglers showed no long-term trend over 50 years, but decreased from 1970 to 1995 and an increase in abundance since 1995. While records of species occurrence were found around much of Great Britain, nearly all were from south of 54° latitude, and records have contracted southwards since 2000. No trend in maximum size through time was detected. In conclusion, we did not find support for the hypothesized declines in abundance and body size or a northward shift in distribution of D. pastinaca and instead found a southward contraction.

Environmental drivers and the distribution of cold-water corals in the global ocean

Species distribution models (SDMs) are useful tools for describing and predicting the distribution of marine species in data-limited environments. Outputs from SDMs have been used to identify areas for spatial management, analyzing trawl closures, quantitatively measuring the risk of bottom trawling, and evaluating protected areas for improving conservation and management. Cold-water corals are globally distributed habitat-forming organisms that are vulnerable to anthropogenic impacts and climate change, but data deficiency remains an ongoing issue for the effective spatial management of these important ecosystem engineers. In this study, we constructed 11 environmental seabed variables at 500 m resolution based on the latest multi-depth global datasets and high-resolution bathymetry. An ensemble species distribution modeling method was used to predict the global habitat suitability for 10 widespread cold-water coral species, namely, 6 Scleractinian framework-forming species and 4 large gorgonian species. Temperature, depth, salinity, terrain ruggedness index, carbonate saturation state, and chlorophyll were the most important factors in determining the global distributions of these species. The Scleractinian Madrepora oculata showed the widest niche breadth, while most other species demonstrated somewhat limited niche breadth. The shallowest study species, Oculina varicosa, had the most distinctive niche of the group. The model outputs from this study represent the highest-resolution global predictions for these species to date and are valuable in aiding the management, conservation, and continued research into cold-water coral species.

Passive ocean acoustic waveguide remote sensing of vocalization behavior and spatial distribution of diverse marine mammal species in the Norwegian and Barents Sea

Leveraging data acquired using a 160-element coherent hydrophone array deployed in the Norwegian and Barents Seas during spring 2014, and the passive ocean acoustic waveguide remote sensing (POAWRS) technique is employed to enable instantaneous wide-area monitoring of marine mammal vocalizations over expanses exceeding 100 km in diameter. The vocalization behavior of diverse marine mammal species including Fin, Humpback, Minke, Sperm, and Beluga whales are analyzed, quantifying time-frequency characteristics and call patterns from their vocalization signals present in high-resolution beamformed power spectrograms. Previously developed automatic detection and machine learning algorithms are employed for clustering and classification of underwater acoustic events, facilitating the subsequent manual audio-visual inspection of whale calls. Bearing-time trajectories of repetitive species-specific vocalizations signals are utilized to estimate locations of the whale calls and hence their temporo-spatial distributions. Through comparative studies, we assess the presence and abundance of marine mammal species-specific vocalization signals in three distinct coastal regions off Norway, namely, Alesund, Lofoten, and Northern Finmark. In addition, we investigate correspondences of marine mammal distributions with concurrently imaged instantaneous wide-area fish distributions from distinct fish species to elucidate potential predator-prey interactions and habitat preferences.

Modeling seasonal changes in the habitat suitability of Coilia nasus in the Yangtze River Estuary using tree-based methods

Estuaries, as highly dynamic marine environments, exhibit seasonal changes in fish habitat requirements, and the selection of appropriate analytical tools is critical to accurately reveal and understand such changes. Tree-based methods (including classification and regression tree (CART), random forest (RF), and conditional random forest (CRF)) have been widely used to explore the habitat requirements and predict the spatial distribution of marine species. However, the optimal methods still need to be comparatively evaluated based on the specific target species and region. This study focused on the important fishery resource Coilia nasus in the Yangtze River Estuary. Based on trawl monitoring survey data from the end of 2013–2018, seven explanatory variables, namely temperature, total nitrogen, latitude, year, salinity, pH, and ammonium, were used as predictors. The area under the receiver operating characteristic curve (AUC), Kappa, true skill statistic (TSS), and three other indicators were used to evaluate the overall performance of the three tree method models. The results showed that (1) almost all indicators pointed to the RF model having the best performance, while CRF and CART were significantly worse for some indicators, and the RF model had the best robustness and was suitable for habitat modeling in all seasons; (2) temperature and total nitrogen constituted the key habitat requirements of C. nasus, and habitat suitability rapidly increased or peaked in the temperature and total nitrogen ranges of 18.1–22.6 °C and 0–0.5 mg/L, respectively; (3) the spatial characteristics of C. nasus habitat in the Yangtze River Estuary exhibited seasonal changes in that the suitable area tended to expand gradually from spring to winter, and the northern branch of the Yangtze River and the offshore area of the Yangtze River were the main areas with high suitability for C. nasus. This study provides a useful reference for the spatial planning of the Yangtze River Estuary fishing ban area and the formulation of related conservation measures in the future.

From the modeling of diadromous species’ marine distributions to the characterization of their current and future marine habitats

IntroductionMany diadromous fish species are currently facing the threat of generalized population extirpations across their range, and conservation efforts implemented primarily within the continental domain may not be sufficient to ensure their viability. In consequence, better understanding the use of the marine environment by these species turned out to be critical.Material and methodWe developed marine species distribution models for two species, Allis shad (Alosa alosa) and the European flounder (Platichthys flesus), and proposed a way to interpret the habitat suitability from a more functional perspective. Our approach defined marine habitats as highly suitable environments that remained with a high potential through the species life stage duration at sea. First, a species-specific probabilistic threshold was applied to select for highly suitable environments each year of the simulations. Then, annual maps of high suitability were combined into recurrence maps showing areas were the environmental conditions remained the most suitable across the life at sea of the given species, these areas being called ‘marine habitats’. We applied this approach to study the current and future marine habitats of Allis shad and European flounder under climate change scenarios.ResultsThe main current marine habitats of these species were coastal, primarily in the Bay of Biscay for Allis shad and in the English Channel-North Sea for European flounder. These habitats are expected to experience minimal changes by mid-century, and the species may even benefit from new habitats at higher latitudes. However, the European flounder is likely to face greater challenges in the central part of its range by the end of the century, as potential spawning grounds may be threatened.DiscussionThis study provided a way to rethink ‘traditional’ suitability maps by integrating basic knowledge on the diadromous species life at sea. Our findings were part of a larger effort to link continental and marine habitats in improving management of diadromous species.

Fishery catch is affected by geographic expansion, fishing down food webs and climate change in Aotearoa, New Zealand.

Historical fishing effort has resulted, in many parts of the ocean, in increasing catches of smaller, lower trophic level species once larger higher trophic level species have been depleted. Concurrently, changes in the geographic distribution of marine species have been observed as species track their thermal affinity in line with ocean warming. However, geographic shifts in fisheries, including to deeper waters, may conceal the phenomenon of fishing down the food web and effects of climate warming on fish stocks. Fisheries-catch weighted metrics such as the Mean Trophic Level (MTL) and Mean Temperature of the Catch (MTC) are used to investigate these phenomena, although apparent trends of these metrics can be masked by the aforementioned geographic expansion and deepening of fisheries catch across large areas and time periods. We investigated instances of both fishing down trophic levels and climate-driven changes in the geographic distribution of fished species in New Zealand waters from 1950-2019, using the MTL and MTC. Thereafter, we corrected for the masking effect of the geographic expansion of fisheries within these indices by using the Fishing-in-Balance (FiB) index and the adapted Mean Trophic Level (aMTL) index. Our results document the offshore expansion of fisheries across the New Zealand Exclusive Economic Zone (EEZ) from 1950-2019, as well as the pervasiveness of fishing down within nearshore fishing stock assemblages. We also revealed the warming of the MTC for pelagic-associated fisheries, trends that were otherwise masked by the depth- and geographic expansion of New Zealand fisheries across the study period.