Fine-scale population structure in a deep-sea teleost (orange roughy, Hoplostethus atlanticus)

Microsatellite and otolith chemistry variability were analysed to assess fine scale genetic structure in the deep-sea teleost orange roughy (Hoplostethus atlanticus). The Porcupine Bank located on the continental shelf west of Ireland, comprises a complex system of mounds and flat areas that are broken up by canyons. Orange roughy form spawning aggregations on mounds and flat areas, and were heavily fished until the resource was depleted. By analysing adults in spawning condition and juvenile orange roughy from six mounds and one flat area, shallow but significant genetic population structure was evident (F_ST=0.0031, D_est across loci=0.0306 and G-test). Most of the structure was accounted for by inclusion of a sample from the flats (six of ten significant pairwise F_ST estimates and G-tests, and five of the highest D_est estimates included the flat sample). While the flat sample contributed most to the genetic structure, there was still significant (albeit weaker) structure among mound samples. The observed structure was supported by otolith analyses. Fish caught as late juveniles in either the flat or mound areas showed consistent differences in chemistry at the otolith core throughout the initial 10 years of growth, which could indicate site fidelity. We hypothesise that seafloor topographic structures (mounds and flats) may provide discrete spawning areas for orange roughy and that the limited gene flow between these spawning areas is insufficient to counteract genetic drift.     

Age and life histories of black and smooth oreos inferred using bomb radiocarbon dating and stable isotope analysis

Black oreo (Allocyttus niger) and smooth oreo (Pseudocyttus maculatus) are commercially exploited deepwater fishes in New Zealand. They are considered to be long-lived with unvalidated maximum otolith growth zone counts of 153 years and 86 years, respectively. Bomb radiocarbon dating results supported the otolith age estimates for A. niger, but those for P. maculatus provided only partial support. Oxygen isotope (δ¹⁸O) results for A. niger otoliths suggested that juveniles were in near-surface waters and adults in depths of 700–1300 m. In contrast, P. maculatus otoliths suggested a variable depth history (500 to at least 1500 m) throughout life. Carbon isotope (δ¹³C) results showed a steady enrichment from core values to c. 20 years for both species, owing to changes in diet and metabolic rate from the juvenile to the adult fish, associated with a depth increase. Von Bertalanffy growth parameters are provided for both species.     

Age verification,growth and life history of rubyfish Plagiogeneion rubiginosum

Age verification of rubyfish (Plagiogeneion rubiginosum) was sought using the bomb radiocarbon chronometer procedure. Stable isotopes were investigated for life history characteristics. Radiocarbon (¹⁴C) and stable isotope (δ¹⁸O and δ¹³C) levels were measured in micro-samples from five otoliths that had been aged using a zone count method. All the core ¹⁴C measurements were ‘pre-bomb’ indicating ages of at least 45 years, and the ¹⁴C measurements across the otolith sections suggested that the zone-count ageing method described herein is not biased. Maximum estimated age was 100 years. There was no significant between-sex difference in the von Bertalanffy growth curves. The δ¹⁸O values indicated that rubyfish are near-surface as juveniles, and move deeper with age. Adults appear to reside in 600–1000 m; this is deeper than most trawl-capture data suggest, but not implausible, and has stock assessment implications. The δ¹³C values reflect fish metabolic rates, trophic feeding levels and oceanographic conditions. The stable isotopes record the environmental life history of each fish, and have value in distinguishing stocks and/or indicating vertical and latitudinal migratory patterns.     

A review of orange roughy Hoplostethus atlanticus fisheries,estimation methods,biology and stock structure

Orange roughy Hoplostethus atlanticus are unusual fish. They form dense aggregations that have fuelled lucrative fisheries at great depths (600–1 400 m), especially off Namibia, New Zealand and Australia. They are thought to be very long-lived (>100 years, maturity at 22 – 40 years), and to have exceptionally low natural mortality (M = 0.045–0.064 year^?1) and slow growth rates (K = 0.055–0.070 year^?1). In addition, they spawn large eggs and have low fecundity. These factors combine to make orange roughy highly susceptible to overfishing; most stocks are below 30% of pristine levels. Assessments are obtained from indices of catch rate and trawl, acoustic and egg surveys. Acoustic estimates are the most direct, but are confounded by the species' low target strength (?50 to ?53 dB)–attributable to the wax-filled swim bladder. Extracellular wax esters are stored in abundance and comprise mostly mono-unsaturated fatty acids, with low concentrations of the ω-3 fatty acid family. This unusual composition (resultant from the species' diet) ensures neutral buoyancy. Stock separation has been inferred mainly from biological studies, but genetic studies have also found differences among stocks within New Zealand and Australia. Deep-water habitat may be damaged by trawling operations and may take many years to recover, so in some quarters there is a call for a portion of suitable habitat to be set aside for preservation. Although Namibian orange roughy are shallower, smaller and younger than those in other stocks, the Namibian fishery sustained high catches for only a few years before quotas were reduced, from 12 000 to 1 875 tons. Three management lessons are suggested for developing orange roughy fisheries based on the Namibian experience: (1) imposition of catch limits during exploratory fishing; (2) starting the acoustic surveys earlier in the fishery, if possible; (3) greater reliance on trends in catch rate until a survey series has been established.     

Impacts of a bottom trawling exclusion zone on Snow Crab abundance and fish harvester behavior in the Labrador Sea, Canada

The Hawke Box trawling exclusion zone off southern Labrador is a 8610 km² closed area implemented in 2003 to protect soft-shelled pre-recruit Snow Crab and juvenile cod from mortality imposed by shrimp trawling and other bottom-impact fisheries. This study utilizes survey, logbook, and observer data from the Snow Crab and Northern Shrimp fisheries to assess the effectiveness of the Hawke Box in protecting and possibly enhancing the Snow Crab resource off southern Labrador. The results indicate that the exclusion zone has failed to protect pre-recruit crabs largely due to a redistribution and intensification of directed Snow Crab effort inside the Box in the years following closure. This unexpected outcome resulted in a high level of fishery-induced mortality to pre-recruit crabs and masked any potential positive outcomes from the cessation of trawling. The shortcomings that contributed to the failure of this exclusion area are discussed in relation to the current state of knowledge on the effectiveness of marine protected areas in general as management tools for conserving aquatic resources.     

Quantifying the relative intensity of fishing on New Zealand seamounts

New Zealand seamounts support major fisheries for several deepwater fish species, including orange roughy (Hoplostethus atlanticus) and smooth oreo (Pseudocyttus maculatus). Although a high proportion of features in the depth range 500–1000 m have been fished, very little is known about the ecological impacts of bottom trawling on seamounts. The potential impact is likely to be influenced by the spatial extent and frequency of fishing. A new index is presented to assess the relative intensity of trawling on New Zealand seamounts. The fishing effects index (FEI) incorporates information on the density of fishing on the seamount as a proportion of the seabed area and also on tow direction. Detailed fisheries data from more than 250 000 tows were examined to calculate FEI for New Zealand seamounts. The most intensively fished seamounts were on the south Chatham Rise, an area characterised by a large number of relatively small features which were fished serially for orange roughy in the 1980s and 1990s. Other seamounts with high FEI were on the north Chatham Rise, Challenger Plateau, and off the east coast of the North Island. A range of sensitivity analyses indicated that the general rankings of seamounts were relatively robust to the choice of arbitrary thresholds used to assign tows to seamounts.     

Tidal variation in a rocky inter‐tidal fish population: the case of white seabream Diplodus sargus juveniles

The rocky inter‐tidal habitat is a harsh and fluctuating environment, subject to frequent disturbances. Field observations of juvenile white seabream Diplodus sargus in inter‐tidal rocky habitats were conducted to analyse the spatial distribution and feeding activity of this species in relation to the tidal cycle. The depth at which fish were observed did not change in most tidal phases while feeding activity changed with tidal level, showing the occurrence of tidal migrations and that feeding may be limited by habitat availability in shallow waters and thus be dependent on tidal changes. The present results show the exploitation of available feeding areas in the rocky inter‐tidal by juvenile white seabream, which corroborates the importance of these habitats for the first developmental stages of this fish species.     

Spatial variability of the surf zone fish and macroinvertebrate community within dissipative sandy beaches in Oregon,USA

Habitat heterogeneity can influence biological communities by providing a diversity of areas that can be occupied by different species. Sandy beach surf zones are often considered homogenous environments; however, sand bars moved by currents and waves can produce trench‐like shapes or troughs that provide heterogeneity. The influence of habitat heterogeneity produced by sand movement is unclear despite the fact that surf zones are an important habitat for larval and juvenile fish and macrocrustaceans. To determine if, and how, the fish and macroinvertebrate communities present in trough and non‐trough or flat areas of Oregon surf zones differ, we compared species assemblages in both areas at three beaches adjacent to estuary mouths over 2 years. Troughs had different communities compared with flat areas, with higher total catch (mean ± SD = 123.2 ± 122.1 versus 43.6 ± 44.5 individuals × 100 m^?2) and taxon richness (6.7 ± 2.7 versus 4.0 ± 2.3 taxa); these differences were potentially due to water movement, prey availability and sediment size. The fish and macroinvertebrate communities did not vary between years but there were significant differences among beaches, with the most distinct community present at the only beach adjacent to an estuary without a jetty at its mouth, which was possibly due to higher species movement between the surf zones and estuary. Fish and macrocrustacean surf zone communities varied spatially within and among beaches in relation to habitat heterogeneity provided by sand movement and, potentially, the influence of adjacent habitats.     

Letter to the editor

It is a common observation that the mean length in a fish population progressively decreases as the biomass is reduced by fishing. However, this has not happened with the population of orange roughy (Hoplostethus atlanticus) on the Chatham Rise, New Zealand, even though the biomass is estimated to have declined by almost 80%. Because orange roughy are slow growing it might be expected that mean age (or its proxy, mean otolith weight) would be a more sensitive indicator of biomass decline than mean length. A simulation study shows that this is not true. With equal sample sizes, all three indicators are equally sensitive; on an equal cost basis, mean length is more sensitive than the others. A log transformation increases the sensitivity of age and otolith weight, but it is not clear whether this increase is sufficient to outweigh the much greater costs of measuring age and otolith weight, compared to length. Under the assumption of constant recruitment, it is more likely than not (P = 0.55–0.8) that a change in mean length would have been detected in the Chatham Rise population, given the estimated biomass decline.     

Community structure and nutrition of deep methane-seep macrobenthos from the North Pacific (Aleutian) Margin and the Gulf of Mexico (Florida Escarpment)

Methane seeps occur at depths extending to over 7000 m along the world's continental margins, but there is little information about the infaunal communities inhabiting sediments of seeps deeper than 3000 m. Biological sampling was carried out off Unimak Island (3200–3300 m) and Kodiak Island (4500 m) on the Aleutian margin, Pacific Ocean and along the Florida Escarpment (3300 m) in the Gulf of Mexico to investigate the community structure and nutrition of macrofauna at these sites. We addressed whether there are characteristic infaunal communities common to the deep‐water seeps or to the specific habitats (clam beds, pogonophoran fields, and microbial mats) studied here, and ask how these differ from background communities or from shallow‐seep settings sampled previously. We also investigated, using stable isotopic signatures, the utilization of chemosynthetically fixed and methane‐derived organic matter by macrofauna from different regions and habitats. Within seep sites, macrofaunal densities were the greatest in the Florida microbial mats (20,961 ± 11,618 ind·m⁻²), the lowest in the Florida pogonophoran fields (926 ± 132 ind·m⁻²), and intermediate in the Unimak and Kodiak seep habitats. Seep macrofaunal densities differed from those in nearby non‐seep sediments only in Florida mat habitats, where a single, abundant species of hesionid polychaete comprised 70% of the macrofauna. Annelids were the dominant taxon (>60%) at all sites and habitats except in Florida background sediments (33%) and Unimak pogonophoran fields (27%). Macrofaunal diversity (H′) was lower at the Florida than the Alaska seeps, with a trend toward reduced richness in clam bed relative to pogonophoran field or non‐seep sediments. Community composition differences between seep and non‐seep sediments were evident in each region except for the Unimak margin, but pogonophoran and clam bed macrofaunal communities did not differ from one another in Alaska. Seep δ¹³C and δ¹⁵N signatures were lighter for seep than non‐seep macrofauna in all regions, indicating use of chemosynthetically derived carbon. The lightest δ¹³C values (average of species’ means) were observed at the Florida escarpment (−42.8‰). We estimated that on average animal tissues had up to 55% methane‐derived carbon in Florida mats, 31–44% in Florida clam beds and Kodiak clam beds and pogonophoran fields, and 9–23% in Unimak seep habitats. However, some taxa such as hesionid and capitellid polychaetes exhibited tremendous intraspecific δ¹³C variation (>30‰) between patch types. Overall we found few characteristic communities or features common to the three deep‐water seeps (>3000 m), but common properties across habitats (mat, clam bed, pogonophorans), independent of location or water depth. In general, macrofaunal densities were lower (except at Florida microbial mats), community structure was similar, and reliance on chemosynthesis was greater than observed in shallower seeps off California and Oregon.     

Fisheries for orange roughy (Hoplostethus atlanticus) on seamounts in New Zealand

Major commercial fisheries for orange roughy (Hoplostethus atlanticus) occur on seamount features, which are widely distributed throughout the New Zealand region. When the fishery developed in the late 1970s to early 1980s, it occurred mainly on flat bottom, but over time has become more focused on seamounts. In the 1995–1996 fishing year, it is estimated that about 70 % of the catch of orange roughy within the New Zealand EEZ was taken from seamounts. Seamounts on the Chatham Rise have been fished for over ten years. Examination of commercial catch and effort data show strong declines in catch rates over time, and a pattern of serial depletion of seamount populations, with the fishery moving progressively eastwards to unfished seamounts along the southern margins of the Rise. Catch rates on seamounts in other regions of New Zealand have also generally shown a similar pattern of rapid decline. There is growing concern over the impact of trawling on seamounts, and the effects this can have on the benthic habitat and fauna, and the long-term sustainability of associated commercial fisheries.     

Trophic relationships along a bathymetric gradient at the deep-sea observatory HAUSGARTEN

Deep-seafloor communities, especially those from the ice-covered Arctic, are subject to severe food limitation as the amount of particulate organic matter (POM) from the surface is attenuated with increasing depth. Here, we use naturally occurring stable isotope tracers (δ¹⁵N) to broaden our rudimentary knowledge of food web structure and the response of benthic organisms to decreasing food supplies along the bathymetric transect (～1300–5600 m water depth) of the deep-sea observatory HAUSGARTEN. Encompassing five trophic levels, the HAUSGARTEN food web is among the longest indicating continuous recycling of organic material typical of food-limited deep-sea ecosystems. The δ¹⁵N signatures ranged from 3.0‰ for Foraminifera to 21.4‰ (±0.4) for starfish (Poraniomorpha tumida). The majority of organisms occupied the second and third trophic level. Demersal fish fed at the third trophic level, consistent with results from stomach contents analysis. There were significant differences in the δ¹⁵N signatures of different functional groups with highest δ¹⁵N values in predators/scavengers (13.2±0.2‰) followed by suspension feeders (11.2±0.2‰) and deposit feeders (10.2±0.3‰). Depth (=increasing food limitation) affected functional groups in different ways. While the isotopic signatures of predators/scavengers did not change, those of suspension feeders increased with depth, and the reverse was found for deposit feeders. In contrast to the results of other studies, the δ¹⁵N signatures in POM samples obtained below 800 m did not vary significantly with depth indicating that changes in δ¹⁵N values are unlikely to be responsible for the depth-related δ¹⁵N signature changes observed for benthic consumers. However, the δ¹⁵N signatures of sediments decreased with increasing depth, which also explains the decrease found for deposit feeders. Suspension feeders may rely increasingly on particles trickling down the HAUSGARTEN slope and carrying higher δ¹⁵N signatures than the decreasing POM supplies, which elevates the δ¹⁵N value of their tissues. Our results imply that a depth-stratified approach should be taken to avoid a misinterpretation of data obtained at different depths.     

New records of cold‐water coral sites and fish fauna characterization of a potential network existing in the Mediterranean Sea

New cold‐water coral (CWC) sites were recorded along the Apulian margin (Central Mediterranean). The species composition and depth distribution of CWCs were updated. A distribution of the CWC sites coincident with the course of the dense‐water masses that flow between the Southern Adriatic and Northern Ionian was confirmed. The faunal assemblages of five of these CWC sites were investigated and compared using experimental longlines during the spring–summer and autumn–winter seasons, between 2010 and 2014. Differences in ecological variables amongst the sites in each season were evaluated by means of a set of univariate and multivariate methods (analysis of variance, permutational multivariate analysis of variance, non‐metric multidimensional scaling). Although some differences were detected in relation to the different depths examined during spring–summer, the CWC sites showed similar features in terms of species richness and diversity as well as in the abundance of the same fish species (Galeus melastomus, Conger conger, Helicolenus dactylopterus, Merluccius merluccius, Phycis blennoides and Pagellus bogaraveo) most probably because of the distribution of adult specimens in structurally complex and heterogeneous habitats, which act as a potential ‘refuge network’ with respect to commercial fishing. The presence of maturing and mature individuals as well as post‐reproductive females indicates that these CWC sites also act as spawning areas, representing a potential ‘renewal network’ for the fish populations. The term ‘network’ used here refers to several similar subsystems (CWC sites) that play the same ecological role in a wider system (Apulian margin). These CWC communities need coherent conservation measures and management strategies according to the Ecosystem Approach to Fisheries.     

Reproductive and feeding spatial dynamics of the black scabbardfish,Aphanopus carbo Lowe, 1839, in NE Atlantic inferred from fatty acid and stable isotope analyses

The black scabbardfish (Aphanopus carbo) is a benthopelagic species widely distributed across the NE Atlantic, where it is admitted to perform a clockwise migration throughout its life cycle stimulated by feeding and reproduction. To overcome the limitations of direct observation of this species, fatty acids profile (FA) and δ¹⁵N and δ¹³C stable isotopes (SI) were analyzed in the muscle tissue of the black scabbardfish and related with diet and maturity. Specimens were collected in four geographic areas in the NE Atlantic: Iceland, the west of the British Isles, mainland Portugal, and Madeira. For all areas, the FA profile was related with the different phases of the reproductive cycle and with diet, whereas the SI were related with diet, environmental characteristics, such as latitude and depth, and particulate organic matter (POM). Stomach content of black scabbardfish caught off mainland Portugal was analyzed and the most frequent prey item identified was the lophogastrid crustacean Gnathophausia zoea, followed by the cephalopod Mastigotheutis spp. and the teleost Rouleina maderensis. For specimens from Iceland and the west of the British Isles, monounsaturated fatty acids (MUFA) were the most important FA, followed by polyunsaturated (PUFA) and saturated FA (SFA), whereas for specimens from mainland Portugal and from Madeira the sequences were PUFA>MUFA>SFA and PUFA>SFA>MUFA, respectively. Immature specimens from the first three areas were found to be accumulating oleic acid which is an intermediate product of the metabolic pathway that transforms SFA to MUFA and these into PUFA. Specimens caught off Madeira were mature and showed a significant prevalence of ARA and DHA which are PUFA with an important role in reproduction. δ¹⁵N was significantly higher in the muscle of black scabbardfish from Madeira, whereas δ¹³C was significantly lower in specimens from Iceland. The low isotopic ratios as well as the prevalence of certain fatty acid trophic markers (FATM) connected specimens from Iceland with small prey. Results indicated that the spatial differences in physiological aspects of this species are related with diet and prey availability in Madeira, mainland Portugal, and the west of the British Isles, as well as variations in the baseline values of the primary production that are related with latitude and depth, mainly in Iceland. The allometric effect of each area׳s size ranges over δ¹⁵N supports the existence of ontogenic differences in the black scabbardfish׳s diet. This diet is typical of mobile benthopelagic predators that are opportunistic feeders.     

Distinctive stable isotope ratios in important zooplankton species in relation to estuarine salinity gradients: Potential tracer of fish migration

To assess the potential of stable isotope ratios as an indicator of fish migration within estuaries, stable isotope ratios in important zooplankton species were analyzed in relation to estuarine salinity gradients. Gut contents from migratory juveniles of the euryhaline marine fish Lateolabrax japonicus were examined along the Chikugo River estuary of the Ariake Sea, which has the most developed estuarine turbidity maximum (ETM) in Japan. Early juveniles in March and April preyed primarily on two copepod species; Sinocalanus sinensis at lower salinities and Acartia omorii at higher salinities. Late juveniles (standard length > 40 mm) at lower salinities preyed exclusively on the mysid Acanthomysis longirostris until July and complementarily on the decapod Acetes japonicus in August. These prey species were collected along the estuary during the spring–summer seasons of 2003 and 2004, and their carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) were evaluated. The δ¹³C values of prey species were distinct from each other and were primarily depleted within and in close proximity to the ETM (salinity < 10); S. sinensis (−26.6‰) < Acanthomysis longirostris (−23.3‰) < Acartia omorii (−21.1‰) < Acetes japonicus (−18.5‰). The overall gradient of δ¹³C with salinity occurred for all prey species and showed minor temporal fluctuations, while it was not directly influenced by the δ¹³C values in particulate organic matter along the estuary. In contrast to δ¹³C, the δ¹⁵N values of prey species did not exhibit any clear relationship with salinity. The present study demonstrated that δ¹³C has the potential for application as a tracer of fish migration into lower salinity areas including the ETM.     

The orange roughy fishery of Namibia: lessons to be learned about managing a developing fishery

Exploration for orange roughy Hoplostethus atlanticus in Namibia started in 1994 and within 12 months several aggregations had been discovered, suggesting the existence of a biomass sufficient to support a viable fishery. At that early stage it was realized that few, if any, recognized management procedures existed for newly developing fisheries, especially with the paucity of data such as existed on Namibian orange roughy. The development of the Namibian orange roughy fishery is reviewed to document the management strategies implemented and how the management of the fishery evolved. The first six years of the fishery are described, including the three-year exploration phase, several years of profitable exploitation, and the severe decline in catch rates. Whether the decline is attributable to fishing mortality or to change in the aggregating behaviour of orange roughy, or both, is not clear. Although many aspects of the precautionary approach were followed, a risk analysis applied and a number of innovative management methods implemented (e.g. incentives to promote exploratory fishing, use of Bayesian statistical methods, implementation of a management plan for long-term total allowable catches), the aggregating biomass declined to between 10 and 50% of virgin levels within the six years. The management methods applied are evaluated in the light of the severe decline in catch rate experienced in 1998 and 1999, so that others may learn from the experience.     

An assessment of juvenile Pacific Ocean perch (Sebastes alutus) habitat use in a deepwater nursery

It is believed that juvenile habitats are important to determining recruitment to adult fish populations through density dependence that occurs in nursery areas. For deepwater marine fishes, the characteristics of nursery areas are generally unknown. The objectives of this study were to examine a potential nursery area for juvenile Pacific ocean perch (POP, Sebastes alutus), determine the specific microhabitats used by juvenile POP, and compare the distribution of juvenile POP to adults. Juvenile POP habitat use was examined at three sites near Samalga Pass in the Aleutian Islands. Presence or absence and density estimates of juvenile POP were made from underwater video collected at 11 transects and from 6 bottom tows at the study sites. Juvenile POP were found predominantly in mixed sand and boulder substrata to the exclusion of most other habitat types. Juvenile POP were found within one body length of complex structure such as boulders, upright coral or sponges. There were higher densities of juvenile POP at the site south of Samalga Pass than at the other sites, while adult POP were found in highest abundance at the site north of Samalga Pass. An examination of large-scale patterns of juvenile and adult POP distribution indicates that juveniles use shallower depth zones on the continental shelf. Combined with the geographic separation we observed in this study, this suggests juvenile POP use nursery habitats that are different from adult POP. Conservation and management of this species should address the habitat requirements of juveniles to maintain the goal of healthy adult populations.     

Reduced abundance of banded kokopu (Galaxias fasciatus) and other native fish in turbid rivers of the North Island of New Zealand

Laboratory experiments demonstrated that migrant juvenile banded kokopu (Galaxias fasciatus Gray) were more sensitive to suspended sediment (SS) than other native fish species. If juvenile migrants avoid waters made turbid by SS and their recruitment to adult habitats up stream is reduced, then adult abundance may decline in turbid rivers. To test this, we compared the abundance of diadromous native fish between turbid and clear rivers. The duration (% time) for which SS concentrations exceeded 120 mg litre^‐1 (a critical level from laboratory experiments) during the migration season (August‐December) was estimated for over 150 New Zealand river sites. Turbid rivers were defined as those where SS concentrations exceeded 120 mg litre^‐1 for over 20% of the time and clear rivers as those where SS concentrations exceeded 120 mg litre^‐1 for less than 10% of the time. Eight turbid rivers and seven clear ones were identified where sufficient data on SS and native fish populations existed to permit a comparison. The mean occurrence of banded kokopu was reduced by 89.5% in turbid rivers and, although other diadromous fish species were also less common, banded kokopu was most affected. Densities of adult banded kokopu were also significantly lower in optimal stream habitats in three turbid compared with three matched clear rivers. We therefore concluded that the abundance of adult banded kokopu was reduced in turbid rivers and propose that this is because of reduced recruitment of juveniles in turbid rivers.     

Carbon stable isotopes in estuarine sediments and their utility as migration markers for nursery studies in the Firth of Forth and Forth Estuary, Scotland

The stable carbon isotope ratios (δ¹³C) of the organic fraction of intertidal sediments in the Forth Estuary and the Firth of Forth, Scotland, were measured to determine if terrestrially derived carbon was present in the estuarine sediments. It was hypothesised that differences in the inputs from marine vs. terrestrial sources to the organic carbon of estuarine and marine sediments, as well as differences in ambient seawater stable oxygen isotope (δ¹⁸O) ratios between the estuary and the Outer Firth, would allow the use of these two stable isotopes as habitat markers for juvenile plaice (Pleuronectes platessa), to allow determination of nursery habitats. Muddy and sandy sediments from the estuary and sandy sediments from the Outer Firth were sampled and δ¹³C measured. Juvenile plaice were caught at two estuarine sites and at two Outer Firth sites and otoliths were removed for δ¹³C and δ¹⁸O analysis. The sandy sediments in the estuary showed a strong gradient of δ¹³C enrichment with distance down the estuary, while the muddy sediments showed a much shallower gradient. δ¹³C and δ¹⁸O measured in the carbonate of juvenile plaice otoliths showed no clear difference between otoliths of fish caught at one of the estuarine sites and at the two Outer Firth sites. However, the isotope ratios of both carbon and oxygen in plaice otoliths from the other estuarine site showed the expected trend of depletion in the heavier isotopes. While the measurements recorded here did not conclusively distinguish between otoliths from juveniles caught in the estuarine site and those caught in the other three sites, they show that stable isotopes have potential to distinguish between estuarine habitats with terrestrial carbon inputs, and coastal marine habitats with predominantly marine carbon inputs.     

Tracing carbon transfer and assimilation by invertebrates and fish across a tropical mangrove ecosystem using stable isotopes

Mangroves are highly productive ecosystems that exhibit a diverse range of habitats, including tidal creeks and flats, forest gaps and interior forest with varying understory light intensity, tidal dynamics, geomorphological settings, and overall biological production. Within mangrove ecosystems, invertebrates and fish feed on heterogeneous food sources, the occurrence of which is unevenly distributed across the system. This provides a basis for testing models of carbon transfer across mangrove ecosystems. We hypothesized that the carbon transfer and assimilation by fish and invertebrates will vary across the different mangrove habitats and that such variations can be predicted by their stable isotope compositions. We analysed δ¹³C and δ¹⁵N signatures of consumers and their potential organic carbon sources across a tropical mangrove ecosystem in Vietnam. The δ¹³C values of crabs and snails significantly decreased from the tidal flat to interior forest, indicating that variations in carbon transfer and assimilation occurred at small scales <30 m. Reduced variation in δ¹³C of suspension‐feeding bivalves suggested that tidal water was a vector for large‐scale transport of carbon across the mangrove ecosystem. An analysis of co‐variance using habitat as a fixed factor and feeding habit and movement capacity of consumers as co‐variates indicated that habitat and feeding types were major features that affected the δ¹³C values of invertebrates and fish. The findings demonstrate that carbon transfer and assimilation across mangrove ecosystems occur as a diverse combination of small (<30 m) and large (>30 m) scale processes.