The relationship between marine fronts and fish diversity in the Patagonian Shelf Large Marine Ecosystem

Aim To evaluate the relationship between marine fronts and (1) fish diversity patterns, (2) fish biomass distribution, and (3) fish assemblage structure. Location Patagonian Shelf, Southwestern Atlantic Ocean. Methods Three main frontal systems and eight nearby zones (frontal vs. non‐frontal areas) were compared. An extensive fishery database, obtained during an a priori sampling scheme (spatial resolution 1 × 1) from 1978 to 1979, was employed. Analyses of 248 trawling stations were performed using a combination of diversity measures (species richness, Shannon diversity and evenness), biomass analyses and multivariate analyses [nonparametric analysis of similarity (ANOSIM), non‐metric multidimensional scaling ordination (NMDS) and similarity percentages (SIMPER)] to evaluate the effect of fronts on fish communities. Results In total, 46 fish taxa were identified. Demersal fish diversity increased at the tidal front of Península Valdés but decreased in the frontal zones of the Southern Shelf‐Break and Magellan frontal systems. The richness and biomass of pelagic fish were higher at the Península Valdés Tidal Front, while the biomass of demersal fish was higher at its non‐frontal counterpart. Fish diversity did not differ consistently along the Northern Shelf‐Break Front. However, a higher fish biomass was registered at the non‐frontal zone. Demersal and pelagic fish assemblages differed between frontal and non‐frontal zones. Main conclusions The Patagonian Shelf Large Marine Ecosystem has diverse frontal types that appear to have different effects on fish diversity and assemblage structure. Seasonal (tidal) or permanent fronts have different effects on the parameters analysed, which could be due to productivity–diversity relationships. The Península Valdés Tidal Front, a relatively short temporal scale system, demonstrates a stronger influence on fish diversity, biomass and assemblage structure than the other frontal areas studied. The seasonality of nutrient input owing to frontogenesis, which begins during the austral spring, may play an important role in determining local diversity. At permanent frontal features, diversity decreases when the nutrient availability is high. This is potentially because, when food resources increase, a few species become dominant.     

Seabird diversity hotspot linked to ocean productivity in the Canary Current Large Marine Ecosystem

Upwelling regions are highly productive habitats targeted by wide-ranging marine predators and industrial fisheries. In this study, we track the migratory movements of eight seabird species from across the Atlantic; quantify overlap with the Canary Current Large Marine Ecosystem (CCLME) and determine the habitat characteristics that drive this association. Our results indicate the CCLME is a biodiversity hotspot for migratory seabirds; all tracked species and more than 70% of individuals used this upwelling region. Relative species richness peaked in areas where sea surface temperature averaged between 15 and 20°C, and correlated positively with chlorophyll a, revealing the optimum conditions driving bottom-up trophic effects for seabirds. Marine vertebrates are not confined by international boundaries, making conservation challenging. However, by linking diversity to ocean productivity, our research reveals the significance of the CCLME for seabird populations from across the Atlantic, making it a priority for conservation action.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Declining Abundance of Beaked Whales (Family Ziphiidae) in the California Current Large Marine Ecosystem

Beaked whales are among the most diverse yet least understood groups of marine mammals. A diverse set of mostly anthropogenic threats necessitates improvement in our ability to assess population status for this cryptic group. The Southwest Fisheries Science Center (NOAA) conducted six ship line-transect cetacean abundance surveys in the California Current off the contiguous western United States between 1991 and 2008. We used a Bayesian hidden-process modeling approach to estimate abundance and population trends of beaked whales using sightings data from these surveys. We also compiled records of beaked whale stranding events (3 genera, at least 8 species) on adjacent beaches from 1900 to 2012, to help assess population status of beaked whales in the northern part of the California Current. Bayesian posterior summaries for trend parameters provide strong evidence of declining beaked whale abundance in the study area. The probability of negative trend for Cuvier''s beaked whale (Ziphius cavirostris) during 1991–2008 was 0.84, with 1991 and 2008 estimates of 10771 (CV = 0.51) and ≈7550 (CV = 0.55), respectively. The probability of decline for Mesoplodon spp. (pooled across species) was 0.96, with 1991 and 2008 estimates of 2206 (CV = 0.46) and 811 (CV = 0.65). The mean posterior estimates for average rate of decline were 2.9% and 7.0% per year. There was no evidence of abundance trend for Baird''s beaked whale (Berardius bairdii), for which annual abundance estimates in the survey area ranged from ≈900 to 1300 (CV≈1.3). Stranding data were consistent with the survey results. Causes of apparent declines are unknown. Direct impacts of fisheries (bycatch) can be ruled out, but impacts of anthropogenic sound (e.g., naval active sonar) and ecosystem change are plausible hypotheses that merit investigation.     

Geographic, seasonal and bathymetric distribution of demersal fish species in the eastern Mediterranean

The distributional abundance of three demersal fish species, Merluccius merluccius, Mullus barbatus and Lophius budegassa, was studied as a function of sampling season, bathymetry and geographic area. Data were collected during research trawl surveys in the Aegean and Ionian seas under the same sampling scheme, thereby allowing comparisons to be made on the mean regional densities in numbers and biomass of these three commercially important species in Greek waters. Results indicated that European hake, M. merluccius, demonstrated a wide bathymetric and geographic distribution, with specimens encountered in all regions between 20 and 500 m depth, although density was found to be highest usually between 101 and 200 m. The mean regional density of hake was observed to vary seasonally, being higher mainly in winter. Red mullet, M. barbatus, was distributed in shallow water depths (<100 m) throughout the Greek seas, particularly in the northern part of the Aegean Sea. Abundance of this species steadily decreased with increasing depth. The geographic distribution of anglerfish, L. budegassa, was found to be restricted to the central and northern part of the Aegean Sea, with the highest mean densities encountered in the Thermaikos Gulf and in intermediate water depths between 101 and 200 m. Spatial patterns of observed density are assumed to be attributed mainly to prevailing topographic and hydrographic conditions and related biological productivity levels.     

Spatiotemporal variability of demersal fish larvae assemblages in the southern region of the California Current

An analysis was conducted of the community structure variability of demersal fish larvae and its relationship with environmental forcing on the west coast of the Baja California Peninsula (25–31°N). The information comes from two data sets of oceanographic cruises in 1997–2000 and 2006–2010. From a total of 190 identified taxa, a high number of species (71) was shared throughout the peninsula. The spatiotemporal contrast in the community structure suggests the separation of two groups (northern and southern) whose boundary is located around Punta Eugenia. The northern group showed an alternate dominance of species with a subarctic-transitional faunistic affinity during the winter and spring and species of a subtropical affinity during summer–autumn. In contrast, the dominant taxa in the southern group have a tropical–subtropical affinity throughout the year, although some taxa of subarctic-transitional affinity also occur during winter–spring. In the spatiotemporal scale, these results suggest that the major cause that modulates the extension and structure of the assemblages is the seasonal oceanic dynamics. Moreover, the intensity of warming or cooling events (El Niño and La Niña) influences the structure of the community on an interannual scale by displacing the zonal boundaries of the groups’ distribution.     

Assessing the roles of population density and predation risk in the evolution of offspring size in populations of a placental fish

Population density is an ecological variable that is hypothesized to be a major agent of selection on offspring size. In high-density populations, high levels of intraspecific competition are expected to favor the production of larger offspring. In contrast, lower levels of intraspecific competition and selection for large offspring should be weaker and more easily overridden by direct selection for increased fecundity in low-density populations. Some studies have found associations between population density and offspring size consistent with this hypothesis. However, their interpretations are often clouded by a number of issues. Here, we use data from a 10-year study of nine populations of the least killifish, Heterandria formosa, to describe the associations of offspring size with habitat type, population density, and predation risk. We found that females from spring populations generally produced larger offspring than females from ponds; however, the magnitude of this difference varied among years. Across all populations, larger offspring were associated with higher densities and lower risks of predation. Interestingly, the associations between the two ecological variables (density and predation risk) and offspring size were largely independent of one another. Our results suggest that previously described genetic differences in offspring size are due to density-dependent natural selection.     

Ecosystem size predicts the probability of speciation in migratory freshwater fish

Predicting speciation is a fundamental goal of research in evolutionary ecology. The probability of speciation is often positively correlated with ecosystem size. Although the mechanisms driving this correlation are generally difficult to identify, a shared geographical and ecological context provides a suitable condition to study the mechanisms that promote speciation in large ecosystems by reducing the number of factors to be considered. Here, we determined the correlation between speciation and ecosystem size, and discuss the underlying mechanisms of this relationship, using a probable parallel ecotype formation for freshwater fish. Our population genetic analysis revealed that speciation of the landlocked goby, Rhinogobius sp. YB, of the Ryukyu Archipelago, Japan, from its migratory ancestor, R. brunneus, occurred in parallel across five islands. Logistic regression analysis showed that speciation probability could be predicted using island size. The results suggest that ecosystem size predicts the occurrence of adaptation and reproductive isolation, probably through its association with three possible factors: divergent selection strength, population persistence, and occurrence probability of habitat separation.     

Territory size and distribution in the African fish eagle

The fish eagles along 32 km of shoreline were counted regularly from a launch. The territory size was found to be 0.6 km of shore with a density of 3.5 eagles per km. A single aerial count along 323 km of shore returned a density of 1.9 eagles per km with a territory size of 1.0 km of shore. The distribution of fish eagles is related to the relative availability of trees as perches and probably also to the abundance of fish. The quantity of fish taken by the fish eagles is shown to be negligible compared either with the total production or with the yield harvested by man.     

The effect of landscape composition and configuration on the spatial distribution of temperate demersal fish

In contrast to the terrestrial environment, where the use of landscape analyses has been clearly demonstrated, the influence of landscape composition and configuration on the abundance and spatial distribution of marine organisms remains poorly understood. Development of this area of marine research has been limited by the lack of accurate benthic habitat maps, particularly for marine environments deeper than can be penetrated by optical remote sensing (<10 m). However, the recent availability of detailed (1:25 000) and accurate habitat maps derived from hydroacoustic surveys of deeper marine waters has redressed this situation. The aim of this research was to establish the strength and significance of derived landscape composition and configuration indices on the demersal fish assemblage structure and spatial distribution. A combination of depth and 6 landscape measures were found to explain 34.8% of the variation in the fish assemblage. Depth contributed the highest percentage of the variance explained, accounting for 23.7%. Distance to reef was the most important landscape index explaining 7.1% of the variability, followed by total length of edge environment (the interface between different benthic substrates) explaining 1.5%. Species responsible for driving the structure of the fish assemblage were also examined and found to respond not only to the proximity of reef but also to the configuration of the reef. For example, Parika scaber and Pseudolabrus psittaculus characterised more heterogeneous landscapes offering clusters of small interconnected patches of reef with a lot of edge environment, whilst juvenile Trachyurus novaezelandiae and Platycephalus caeruleopunctatus preferred more homogeneous landscapes offering little reef or edge environment. This research demonstrated that a broad scale landscape analysis, employing indices of landscape composition and configuration, is important for understanding demersal fish assemblage structure and spatial distribution.     

Spatial distribution and diversity of commercial demersal fish species in the shelf waters of the Caspian Sea

The Caspian Sea (CS) is a unique ecosystem known for its several fish species, especially sturgeons. To exhaustively manage the fish stocks of this ecosystem, detailed knowledge of species composition, abundance, distribution, and the habitat traits of the living organisms is necessary. This study analyzed the diversity, spatial, and seasonal distribution of commercial demersal fish species, and examined the relationships between community structure and environmental variability in Iranian shelf waters of the CS. For this purpose, seasonally fish sampling took place between 2009 and 2011 with a bottom trawl. Among 11 fish species captured, Chelon spp., Rutilus kutum, and Vimba vimba showed the highest abundances (i.e. 88.49%, 10.67%, and 0.69%, respectively). Univariate and multivariate analyses showed differences in commercial fish assemblages (abundance and species richness) according to seasons and regions. Distance-based Linear Model (DisTLM) showed that eight environmental variables display significant linear relationships with the fauna resemblance matrix (p < .05). Based on AIC criteria, the combination of silt&clay, TOM, longitude, depth, bottom, and surface temperature used to build the parsimonious DisTLM model explain 67.03% of the total variability. The results revealed a biogeographical and temporal gradient from the west to the east and summer to winter, in terms of commercial demersal fish assemblages and species diversity, as a consequence of different geomorphological, bottom substratum conditions and benthic communities.     

A simulation-approach to assess the size structure of commercially exploited fish populations within the European Marine Strategy Framework Directive

Assessing the size structure and composition of fish stocks, as demanded by the Marine Strategy Framework Directive (MSFD), is considered to be critical for the evaluation of the health status of exploited populations. The MSFD explicitly contains the concept that many large individuals within a population are indicative of a healthy stock. To reflect the abundance of large individuals, the EU-Commission suggested several size-based indicators (SBI). Only few of these SBI have been tested within a pressure-state relationship, in which a state indicator is sensitive, responsive and specific to a given pressure. Sensitivity and responsiveness of pressure-state relationship can be validated by cross-correlating time-series of pressure and state indicators. In the real world, however, time-series of ecological indicators are not only affected by the influence of a pressure, but by natural variability, changes in the sampling method and stochasticity. Hence observed cross-correlations between pressure and state indicator time-series may not be based on a true causal link, especially if time-series are short (less than 30 years). To overcome these limitations, the performance of eight SBI was tested with a population model in which pressures (fishing mortality) and states of ecosystem components (SBI) were precisely known. We distinguished between relative SBI reflecting proportions of size-classes and absolute SBI reflecting absolute entities of the size-distribution such as the observed maximum size in a given survey year. Relative SBI were more sensitive to recruitment than absolute SBI, but not to fishing pressure, which makes relative SBI unsuitable for the assessment of the abundance of large individuals within a population. The outcomes of the model simulation were confirmed by a case study on North Sea cod, where the mean length of the largest ten individuals caught in a given survey year (L_max10) emerged as a promising indicator for the assessment of size structure. Exemplary, we demonstrate how the L_max10 may be incorporated into the existing International Council for the Exploration of the Sea (ICES) assessment framework by defining target and limit values of the good environmental status.     

A mathematical derivation of size spectra in fish populations

Taking into account a predator/prey size ratio in a size-structured population model leads to a partial derivative equation of which we study the properties. By expliciting the structure of the attractor of this equation, it is shown that a simple mechanism, size-based opportunistic predation, can explain the stability in the shape of size spectra observed in various marine ecosystems.     

Changes in species diversity and size composition in the Firth of Clyde demersal fish community (1927-2009)

Following the repeal in 1962 of a long-standing ban on trawling, yields of demersal fish from the Firth of Clyde, southwest Scotland, increased to a maximum in 1973 and then declined until the directed fishery effectively ceased in the early 2000s. Since then, the only landings of demersal fish from the Firth have been by-catch in the Norway lobster fishery. We analysed changes in biomass density, species diversity and length structure of the demersal fish community between 1927 and 2009 from scientific trawl surveys, and related these to the fishery harvesting rate. As yields collapsed, the community transformed from a state in which biomass was distributed across numerous species (high species evenness) and large maximum length taxa were common, to one in which 90 per cent of the biomass was vested in one species (whiting), and both large individuals and large maximum length species were rare. Species evenness recovered quickly once the directed fishery ceased, but 10 years later, the community was still deficient in large individuals. The changes partly reflected events at a larger regional scale but were more extreme. The lag in response with respect to fishing has implications for attempts at managing a restoration of the ecosystem.     

The effects of long-term exploitation of demersal fish populations off the coast of Sierra Leone, West Africa

The present condition of the inshore sciaenid fauna in Sierra Leone, based on the data collected in 1985–86 from commercial inshore trawlers and accumulated records, is compared with the first assessment of the virgin stocks conducted in the 1950s. This has enabled the effects of the long term on this tropical multispecies fishery to be assessed. In addition to the changes in the species composition, the estimation of growth and mortality from both time periods have allowed the population characteristics of the most common dermersal species to be compared.     

Position of egg within a clutch is linked to size at hatching in a demersal tropical fish

Size variation among propagules is ubiquitous and small initial differences in size can be critical to survival, particularly in taxa where initial survival is variable and strongly size-dependent. Despite this, the sources of size variation among fish at hatching are rarely investigated. This study examined spatial position within egg clutches as a source of size variation at hatching of the benthic spawning fish Amphiprion melanopus. We quantified within-clutch size variation at hatching and found that newly hatched larvae from the periphery (5 mm from edge) of 2-dimensional clutches were smaller in standard length, cranial depth, eye diameter and body area (7%, 8%, 4% and 11%, respectively) than larvae from the interior positions within clutches. To investigate the source of this variation, sizes of embryos at different locations with clutches were measured within 2 h of fertilisation (8 d before hatching). Newly laid embryos from the clutch periphery were smaller in length and volume than embryos from the clutch interior (> 2% and 4-6%, respectively). These eggs from the periphery also had a 33% lower rate of oxygen consumption than did embryos from the clutch interior, throughout development. The relationships between position within a clutch and egg size, oxygen consumption and larval size imply that size variation in larval fish at hatching is partly generated during early embryogenesis, either from maternal endowment or maternal nest design, and was amplified throughout development.