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.     

Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine

A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km²/day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring.

The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod.     

Discrimination between Icelandic cod (Gadus morhua L.) populations from adjacent spawning areas based on otolith growth and shape

Studies on Atlantic cod (Gadus morhua L.) have demonstrated extensive variation in life history characteristics among populations exposed to different environmental conditions. In Iceland, cod sampled from adjacent spawning areas, within the main spawning area at the southwest coast, exhibit variation in vital life history characteristics. In this study, we investigated the stock structure of Icelandic cod on the main spawning grounds to discriminate between these adjacent spawning groups by using otolith growth and shape. Otolith growth was based on annual increment width estimated for the major age groups (6–7 year olds) of the spawning stock. Otolith shape was determined using Fourier analysis and compared among cod sampled from the different spawning areas with stepwise canonical discriminant analysis (CDA). We found significant differences in growth and otolith shape between adjacent spawning groups of cod where those sampled close to the coast differed from those that were sampled further out on the bank and continental shelf. Our results suggest that these cod may belong to different populations and the large and fast growing cod spawning in the coastal area may need special protection given their significance to the overall productivity of the stock.     

Discrete spatial dynamics in a marine broadcast spawner: Re-evaluating scales of connectivity and habitat associations in Atlantic cod (Gadus morhua L.) in coastal Newfoundland

We examined the spatial dynamics of Atlantic cod, Gadus morhua, through its life history on the northeast and southeast coasts of Newfoundland in order to delineate dispersal and critical habitat requirements as potential determinants of appropriate spatial scales for management. Hydroacoustic surveys across the northeast Newfoundland shelf in 1990, 1995, and 2000 showed the distribution of the remaining large aggregations of cod to be concentrated in inshore embayments, which is contrary to historic patterns. Inshore surveys on the south coast identified local spawning aggregations at specific geographic locations, to which adults homed in subsequent years. Dispersal of eggs and larvae to the offshore occurred during spring, but was localized (within bay) in the warm summer months. Abundance of age-0 cod increased with proximity to spawning location inshore, suggesting self-recruitment within local embayments. Age-2–3 juveniles actively selected structurally complex habitat at scales of less than tens of kilometers, with home ranges typically less than 10 ha. Juveniles were associated with specific habitat types, especially at age 0, in both lab and field experiments. Our results suggest that self-recruitment and critical life-history events (e.g., spawning, settlement, and maturity) may occur within local embayments, which currently assume much greater importance to overall cod stock dynamics than in the past. Given evidence for limited dispersal, and the inshore presence of critical habitat for early life-history stages, spatial management tools that consider local dynamics may provide a greater degree of protection for coastal Newfoundland cod than previously thought.     

Using a biophysical model to investigate connectivity between spawning grounds and nursery areas of King George whiting (Sillaginodes punctatus: Perciformes) in South Australia’s gulfs

Many demersal marine fish species depend on a dispersive larval stage that connects geographically discrete sub‐populations. Understanding connectivity between these sub‐populations is necessary to determine stock structure, which identifies the appropriate spatial scale for fishery management. Such connectivity is poorly understood for King George whiting (Sillaginodes punctatus; Perciformes) in South Australia's gulf system, even though spawning grounds and nursery areas are adequately defined. In response to declines in commercial catches and estimated biomass, this study aimed to determine the most important spawning grounds and nursery areas to recruitment, and the connectivity between them. A biophysical model was seeded with particles according to the distribution and density of eggs throughout the spawning area in 2017 and 2018. Despite inter‐annual differences in the origins of particles, dispersal pathways and predicted settlement areas remained consistent between years. Predicted settlement was generally highest to nursery areas only short distances from regional spawning grounds, consistent with previous hydrodynamic models. However, the model also predicted that spawning in one region could contribute to recruitment in an adjacent region later in the spawning season, which aligned with the breakdown of thermohaline fronts at the entrance of each gulf. The connectivity between spawning grounds and nursery areas predicted by the model is supported by spatio‐temporal patterns in the otolith chemistry of pre‐flexion larvae and settled juveniles. Consequently, the most parsimonious explanation is that the populations of King George whiting in South Australia's gulf system constitute a single, panmictic stock, which has implications for fishery management.     

Age, growth, and reproduction of silky sharks, Carcharhinus falciformis, in northeastern Taiwan waters

To examine the age, growth and reproduction of silky sharks, Carcharhinus falciformis, in the waters off northeastern Taiwan, 469 specimens (213 females and 256 males) were collected from August 2000 to January 2002 at the Nanfanao fish market, northeastern Taiwan. The relationship between body weight (W) and total length (TL) for both sexes combined was expressed as: W = 2.92 × 10⁻⁶ TL^3.15 (n = 469, p < 0.01). The relationship between TL and vertebral centrum radius (R) for both sexes combined data was estimated as: TL = 25.979 + 18.197R (n = 250, p < 0.01). Growth bands (including translucent and opaque zones) in precaudal vertebrae formed once a year between December and January and were counted up to 11 and 14 for females and males, respectively. The von Bertalanffy growth function (VBGF) was used to model the observed length at age data. The sexes combined VBGF predicted an asymptotic length (L_∞) = 332.0 cm TL, growth coefficient (k) = 0.0838 year⁻¹, age at zero length (t₀) = −2.761 year (n = 250, p < 0.01). Size at 50% maturity for males was estimated to be 212.5 cm based on the logistic curve, which corresponded to 9.3 years. Females matured at 210–220 cm, which correspond to 9.2–10.2 years. The length at birth was estimated to be 63.5–75.5 cm TL. The number of embryos per litter was 8–10 and sex ratio of embryos was 1:1.     

Spatiotemporal distribution of Atlantic cod (Gadus morhua L.) with intact and blocked olfactory sense during the spawning season in a Norwegian fjord with intensive salmon farming

In several areas along the Norwegian coast local fishermen claim that wild migrating Atlantic cod (Gadus morhua L.) have changed their spawning migratory behaviour following establishment of salmon farms, and no longer enter their natural spawning grounds in the fjords. This has created a heated debate in Norway and, in some areas, passed down a moratorium on establishing new salmon farms. Research has so far not been able to show a causal connection, but it has been suggested that water-soluble odorants from high density salmon stocks in commercial fish farms might be responsible for the avoidance of areas with fish farming and abandonment of nearby traditional spawning grounds. The aim of this study was to test this hypothesis by studying the spatiotemporal distribution of wild migrating cod tagged with acoustic transmitters using an array of automatic listening stations in Øksfjord, a Norwegian fjord with intensive fish farming and traditional spawning grounds for cod. The tagged cod were released either in the outer parts of Øksfjord, i.e. in the area they were caught, or in the inner part of the fjord close to their traditional spawning areas. The olfactory sense was physically blocked for 50% of the fish. Only one cod released in the outer part of the fjord visited the traditional spawning grounds. The majority of the cod that were released in the inner part of the fjord remained in this area for approximately three weeks before they left the fjord. There were no differences in spatiotemporal distribution between cod with intact and cod with blocked olfactory sense. Thus, the results provided no evidence that migrating wild cod avoid areas with fish farming as a response to odorants, but the results do not per se contradict the fishermen's observation that coastal cod have changed their migratory pattern in this fjord.     

Oceanographic influences on the distribution of Baltic cod, Gadus morhua, during spawning in the Bornholm Basin of the Baltic Sea

The Baltic Sea is a stratified, semienclosed sea typified by a low-salinity surface layer and a deep saline layer of varying volume, salinity, temperature and oxygen concentration. The relationships between these oceanographic factors and the distribution of Baltic cod are presented, utilizing results from a survey carried out during the 1995 spawning period in the Bornholm Basin, at present the main spawning area of this stock. Cod distribution, abundance and population structure were estimated from hydroacoustic and trawl data and related to hydrographic parameters as well as to bottom depth. In the central basin, cod were aggregated in an intermediate layer about 15 m thick. This area of peak abundance was defined at its upper limit by the halocline and at the lower limit by oxygen content. The majority of individuals caught in the basin centre were in spawning or pre-spawning condition with a high proportion of males to females. On the basin slopes, aggregations of cod were found near the bottom. These individuals were mainly immature and maturing stages with an increasing proportion of females to males with size. Salinity and oxygen conditions were found to be the major factors influencing the vertical and horizontal distribution of adult cod. Abundance of immature cod was also positively related to decreasing bottom depths. The effect of temperature was minor. The observed size- and sex-dependent spawning aggregation patterns, in association with habitat volume and stock size, may influence cod catchability and thereby the assessment and exploitation patterns of this stock.     

A centurial development of the North Sea fish megafauna as reflected by the historical Swedish longlining fisheries

Historically, to compensate for declining catches, fishers have usually shifted from species characterized by high catch rate onto less easily caught species or have moved into new fishing grounds. Such shifts are poorly documented for areas with a long history of exploitation (i.e. North Sea) as they occurred long time before the start of the regular assessments of the marine resources. The Swedish longline fisheries in the Kattegat‐Skagerrak and North Sea have a long history that spans over several centuries. These fisheries have historically targeted large demersal predator fish as ling (Molva molva), cod (Gadus morhua), Atlantic halibut (Hippoglossus hippoglossus) and skates (mainly Dipturus spp.). In this study, data from the Swedish longline fisheries from 1859 to 1960 have been collated. The data show that the geographical expansion of the fishery was extensive. At the turn of the 20th century, offshore longlining became concentrated north and west of the Shetlands and Hebrides, and after the WWII, the fishery expanded to Iceland and Rockall. In the offshore fishery, CPUE for the main target species, ling, remained stable, whereas for the other species, with the exception of tusk (Brosme brosme), CPUE showed a dramatic decline over time. In contrast, in the coastal longlining fishery, severe declines were revealed for all major target species except cod. We argue that the constant search for new fishing grounds in the Northeast Atlantic reflects a dwindling resource, where the fishermen kept the catch rates of ling high by travelling to more and more distant fishing grounds.     

Movements and spatiotemporal distribution of escaped farmed and local wild Atlantic cod (Gadus morhua L.)

Commercial farming of Atlantic cod (Gadus morhua L.) is now being developed in several countries. The ecological consequences of cod culture are poorly understood, but recent research suggests that Atlantic cod are more prone to escape from net pens than Atlantic salmon. Here, we describe the movements and the spatiotemporal distribution of farmed cod after escape relative to wild cod, both during and outside the natural spawning season. The experimental design included simulating escape incidents of farmed cod tagged with acoustic transmitters and using an array of automatic listening stations to monitor their dispersal and distribution. For comparison, local wild cod were monitored using the same array of receivers. The farmed cod dispersed rapidly after a simulated escape, they randomly distributed over large areas and their distribution overlapped with local wild cod. Moreover, escaped farmed fish were found at local cod spawning areas during the spawning season. The study also indicated that the recapture rate of escaped farmed cod was high compared with that of escaped farmed salmon. Thus, while our results showed that there is a considerable potential for ecosystem effects caused by escaped farmed cod, mitigating actions such as an efficient recapture fishery for escapees may be possible.