Dispersal of Eastern King Prawn larvae in a western boundary current: New insights from particle tracking

Patterns in larval transport of coastal species have important implications for species connectivity, conservation, and fisheries, especially in the vicinity of a strengthening boundary current. An Ocean General Circulation Model for the Earth Simulator particle tracking model was used to assess the potential dispersal of Eastern King Prawn (EKP) larvae Melicertus (Penaeus) plebejus, an important commercial and recreational species in Eastern Australia. Particles were exposed to a constant natural mortality rate, and temperature‐dependent growth (degree‐days) was used to determine the time of settlement. Forward and backward simulations were used to identify the extent of larval dispersal from key source locations, and to determine the putative spawning regions for four settlement sites. The mean dispersal distance for larvae was extensive (~750–1,000 km before settlement), yet the northern spawning locations were unlikely to contribute larvae to the most southern extent of the EKP range. There was generally great offshore dispersal of larvae, with only 2%–5% of larvae on the continental shelf at the time of settlement. Our particle tracking results were combined with existing site‐specific reproductive potentials to identify the relative contributions of larvae from key source locations. Although mid‐latitude sites had only moderate reproductive potential, they delivered the most particles to the southern coast and are probably the most important sources of larval EKP for the two southern estuaries. Our modelling suggests that mesoscale oceanography is a strong determinant of recruitment success of the EKP, and highlights the importance of both larval dispersal and reproductive potential for understanding connectivity across a species’ range.     

Modeled transport of winter flounder larvae spawned in coastal waters of the Gulf of Maine

Winter flounder, Pseudopleuronectes americanus, from southern New England and the Gulf of Maine were historically considered to be obligate estuarine spawners. However, recent experiments and observations document that winter flounder in the Gulf of Maine also utilize coastal waters for spawning. An individual‐based modeling approach was used to investigate the transport of winter flounder larvae from three hypothesized coastal spawning grounds in the Gulf of Maine. Transport success rates were greatest for larvae released from Ipswich Bay, intermediate for Stellwagen Bank and least successful for those released from Jeffreys Ledge. There was substantial interannual variability in larval transport and geographic patterns of potential connectivity. Furthermore, the date of spawning had an important influence on transport success. Model results suggest that certain coastal spawning grounds used by winter flounder may serve as an important source of larvae to estuaries and nearshore nursery areas. The potential influx of coastal spawned larvae could have implications for the resilience, productivity and gene flow in local populations. Model results provide further support for the conclusion that winter flounder in the Gulf of Maine may not be solely dependent upon estuaries for spawning. Results also suggest that coastal spawning groups should be considered explicitly in the management of winter flounder, and protected under Essential Fish Habitat regulations.     

Using underwater cameras to describe the reproductive behaviour of the endangered eastern freshwater cod Maccullochella ikei

Abstract –  Underwater cameras were used to observe the breeding behaviour of the endangered eastern freshwater cod, Maccullochella ikei , over 3 years and across three areas in the Mann and Nymboida rivers, Australia. The annual breeding season for M. ikei was short and succinct, lasting only 8–10 weeks. Spawning commenced each year in the lowest altitude area during the first week of spring, and approximately 1 week later in the closest upstream area. Day-length is the primary spawning cue for M. ikei , but increasing water temperature may also be of importance. Nesting sites were located in slow-flowing pools, under cover such as large boulders and bedrock shelves, at depths of 0.9–4.0 m, and with one or two entrances only. The nesting site was vigorously cleaned by the male up to 1 week prior to spawning and was only entered by the female for spawning. Paternal care of eggs and larvae was undertaken for up to 24 days, after which larvae dispersed. Greater protection of breeding M. ikei must be a management priority to ensure long-term conservation of the species.     

Larval lake sturgeon production and drift behaviour in the Menominee and Oconto Rivers,Wisconsin

Lake sturgeon (Acipenser fulvescens) have been negatively impacted by barriers to migration, pollution and overharvest. Biological data such as the timing of spawning activity and larval drift have provided a better understanding of larval production success or failure on a site-specific basis. However, many river systems that serve as reproductive locations for remnant lake sturgeon populations remain understudied. The objective of this study was to compare and contrast aspects of larval lake sturgeon drift within and between the Oconto and Menominee Rivers over multiple reproductive seasons. Differences in larval drift chronology, the size of drifting larvae during the season and the cross-sectional profile of drifting individuals as a function of river velocity were evaluated. D-frame drift nets were deployed at a single transect below spawning sites in the Menominee (2012, 2013 and 2014) and Oconto Rivers (2013, 2014 and 2015). A total of 4,442 larvae were captured across all seasons. Larvae exhibited nonrandom drift profiles related to water velocity, and larvae size was related to horizontal drift location. The seasonal increase in body size of drifting larvae also differed among drift events within and between years and across rivers. This information will provide a better understanding of lake sturgeon early life history and has practical management connections including spawning site protection/enhancement and river flow mitigation.     

The critical importance of an undammed river segment to the reproductive cycle of a migratory Neotropical fish

Biotelemetry, ichthyoplankton and genetic data can provide detailed information about the migratory dynamics and reproductive cycle of freshwater fishes. However, few studies have combined these techniques in Neotropical systems. The objective of this study was to examine the migratory and reproductive dynamics of Prochilodus costatus in the São Francisco River watershed, south‐east Brazil, by comparing the ecological importance of two rivers to the species, an undammed segment of the São Francisco River and a dammed segment of one of its main tributaries, the Pará River. In total, 215 fish were radio‐tagged over three years (2014–16). Eggs and larvae were sampled at seven locations and analysed by PCR to identify Prochilodus spp. ichthyoplankton. Most radio‐tagged individuals (97%) used the undammed segment of the São Francisco River as spawning migration route, even those captured and released in the Pará River. Fish migrated to spawn from late September to late November with the arrival of the rains and returned to feeding sites from December to May after spawning. The highest densities of fish eggs and larvae were recorded in the upper reaches of the São Francisco River during months of peak river discharge. Returning fish showed high fidelity to sites occupied before spawning migration. Fish spent roughly 71% of the year at feeding sites, 25% at spawning sites and 4% moving between them. This study provides novel information about the migratory dynamics of Neotropical fishes and underscores the key role of undammed river segments for the conservation of Neotropical migratory fish species.     

Advection of anchovy (Engraulis encrasicolus) larvae along the Catalan continental slope (NW Mediterranean)

The Gulf of Lions is one of the main anchovy (Engraulis encrasicolus) spawning areas in the NW Mediterranean. During the spring, low‐salinity surface water from the outflow of the Rhône is advected by the shelf‐slope current along the continental slope off the Catalan coast. In June 2000, a Lagrangian experiment tracking these low‐salinity surface waters was conducted to assess the importance of this transport mechanism for anchovy larvae and to determine the suitability of the tracked surface waters for survival of anchovy larvae. The experiment consisted of sampling the tracked water parcel for 10 days with three drifters launched at the core of the shelf‐slope current where low‐salinity surface waters were detected. The survey was completed by sampling the surrounding waters. Anchovy larvae from the spawning area in the Gulf of Lions were advected towards the south in the low‐salinity waters. The size increase of anchovy larvae throughout the Lagrangian tracking closely followed the general growth rate calculated by otolith analysis (0.65 mm day⁻¹). However, advection by the current was not the only mechanism of anchovy larval transport. A series of anticyclonic eddies, originated in the Gulf of Lions and advected southwards, seemed to play a complementary role in the transport of larvae from the spawning ground towards the nursery areas. These eddies not only contributed to larval transport but also prevented their dispersion. These transport and aggregation mechanisms may be important for anchovy populations along the Catalan coast and require further study.     

Modelling transport of inshore and deep‐spawned chokka squid (Loligo reynaudi) paralarvae off South Africa: the potential contribution of deep spawning to recruitment

The South African chokka squid, Loligo reynaudi, spawns both inshore (≤70 m) and on the mid‐shelf (71–130 m) of the Eastern Agulhas Bank. The fate of these deep‐spawned hatchlings and their potential contribution to recruitment is as yet unknown. Lagrangian ROMS‐IBM (Regional Ocean Modelling System‐Individual‐Based Model) simulations confirm westward transport of inshore and deep‐spawned hatchlings, but also indicate that the potential exists for paralarvae hatched on the Eastern Agulhas Bank deep spawning grounds to be removed from the shelf ecosystem. Using a ROMS‐IBM, this study determined the transport and recruitment success of deep‐spawned hatchlings relative to inshore‐hatched paralarvae. A total of 12 release sites were incorporated into the model, six inshore and six deep‐spawning sites. Paralarval survival was estimated based on timely transport to nursery grounds, adequate retention within the nursery grounds and retention on the Agulhas Bank shelf (<200 m). Paralarval transport and survival were dependent on both spawning location and time of hatching. Results suggest the importance of the south coast as a nursery area for inshore‐hatched paralarvae, and similarly the cold ridge nursery grounds for deep‐hatched paralarvae. Possible relationships between periods of highest recruitment success and spawning peaks were identified for both spawning habitats. Based on the likely autumn increase in deep spawning off the Tsitsikamma coast, and the beneficial currents during this period (as indicated by the model results) it can be concluded that deep spawning may at times contribute significantly to recruitment.     

Genetic differentiation between and within ecotypes of pike (Esox lucius) in the Baltic Sea

1. Aquatic systems often lack physical boundaries for gene flow, but ecological and behavioural barriers can form surprisingly fine spatial scale genetic patterns that challenge traditional, large scale management. To detect fine spatial scale structures, understand sources of intraspecific diversity, and design appropriate management plans requires identification of reproductively isolated units.
2. This study reports on genetic differentiation in pike (Esox lucius) within a coastal area stretching 55 km from south to north in the central Baltic Sea. Pike is here an economically and ecologically important top predator that has declined in abundance. However, population structures have mostly been studied on large spatial scales, and without considering the potential for genetic divergence between the sympatric anadromous fresh water and the resident brackish water spawning ecotypes.
3. To this end, 487 individuals from the east coast of Sweden and the island of Öland, representing sympatric anadromous and resident spawning individuals, categorized to ecotype based on spawning location or otolith microchemistry, were genotyped for 10 microsatellites and used to test for divergence between ecotypes. Furthermore, divergence between regions (island/mainland), neighbouring spawning locations (n = 13) and isolation by distance within and between regions were evaluated for the anadromous ecotype.
4. The results revealed strong genetic differences between regions, between spawning locations separated by as little as 5 km and the first evidence of genetic differentiation between resident and anadromous ecotypes; despite a high dispersal capacity of pike and a high connectivity within the study area. The signatures of isolation by distance indicated that connectivity among populations differed between regions, probably reflecting availability of spawning habitats.
5. To safeguard against the challenges and uncertainties associated with environmental change, adaptive conservation management should aim to promote high intra‐population functional genetic diversity without compromising the continued integrity and coexistence of the different ecotypes and of locally adapted sub‐populations.

     

Recent decline in cod stocks in the North Sea–Skagerrak–Kattegat shifts the sources of larval supply

Cod stocks in the North Sea, including the Kattegat and the Skagerrak, have declined dramatically since the 1970s. Occasionally there is a high recruitment of juveniles in Kattegat/Skagerrak, without leading to the rebuilding of adult cod stocks despite reduced fishing mortality. In a biophysical model of egg and larval drift, we examined the potential importance of extant and historical spawning grounds for recruitment of cod in the Kattegat/Skagerrak seas using data of spawning stock biomass from the 1970s and from today's reduced stocks. The results suggest that Kattegat in the 1970s relied on largely locally retained (83%) larvae with little annual variation in recruitment. Kattegat also provided a substantial proportion of larvae recruiting in Swedish Skagerrak (72%). This is in contrast to present conditions where the Kattegat spawning stock has been reduced by 94%, and Kattegat only provides 34% of locally retained larvae and 30% to Swedish Skagerrak. Instead, the protected area in the Öresund and the Belt Sea are expected today to provide most larvae recruiting in Kattegat. Also, the inflow of larvae from the North Sea to Skagerrak and Kattegat can be significant although highly variable between years, with a positive correlation to the North‐Atlantic Oscillation index (NAO). The rebuilding of healthy spawning areas in the Kattegat may be key for restoring local cod stocks in both Kattegat and along the Skagerrak coast. This poses a management challenge if cod with local ‘Kattegat’ adaptations, e.g., in terms of egg density and migration patterns, are lost or reduced to non‐resilient densities.     

Larval population structure of Engraulis encrasicolus in the Strait of Sicily as revealed by morphometric and genetic analysis

In order to investigate the larval population structure, specimens of Engraulis encrasicolus larvae from five different locations in the Strait of Sicily were analyzed by means of otolith readings, morphometric and genetic techniques. The distribution of age in day, identified by means of the otolith readings and associated with the oceanographic parameters, was useful to identify possible spawning areas and transport dynamics. The presence of more than one spawning area suggested the possibility that two or more sub‐populations may co‐exist in the study area. The morphometric characteristics were more adequate than genetic parameters to discriminate the different larval groups. The most relevant variables for the separation were the mouth length (ML) and the body diameter (BD). The population structure by means of genetic data reported the presence of two phylogroups co‐occurring among samples in each sampling locations specimens. The pattern of genetic divergence among anchovy larvae in the Strait of Sicily was congruent with previous studies conducted on adult populations present in other Mediterranean areas with different molecular markers. The habitat‐specific nature of the morphological variation and the lack of corresponding genetic variation among larvae from the different locations suggested that the observed differences in morphology could be linked to environmental parameters. The body form differences among different larvae samples could reflect the nutritional status of larvae. In fact, these differences were found among anchovy larvae collected in areas with different oxygen and fluorescence, which is an index of primary productivity and is linked to the availability of food for anchovy larvae.