Juvenile Gadoids Research Articles

Habitat associations of juvenile Atlantic cod (Gadus morhua L.) and sympatric demersal fish communities within shallow inshore nursery grounds

Resolving the relationship between demersal fish and sublittoral biotic habitats and substrates is a key element in the protection of important nursery grounds for strengthening fish recruitment. In Norway, coastal Atlantic cod (Gadus morhua L.) is a commercially and culturally important demersal fish for Norwegian coastal communities, and in recent decades the stock has declined to such an extent that a plan to rebuild the stock to biologically safe limits has been implemented. Yet, little is known about the specific biotic and abiotic habitat associations of the early-life stages of coastal cod, which is important for the management and protection of the species. The same shallow, sublittoral zones are inhabited by juveniles and adults of other commercial demersal gadoids and wrasses. This study presents novel findings on associations between juvenile coastal cod and other demersal fish species with seafloor substrates and biological habitats, inferred from five years of extensive fyke net surveys. Newly settled 0-group cod were typically associated with eel grass and red algae biotic habitats on sand and shell sand substrates. However, there was an ontogenic habitat shift amongst one year old (1-group) individuals that became more ubiquitous with their biotic habitat or substrate type choices. The juvenile gadoids saithe (Pollachius virens) and pollack (Pollachius pollachius), were most associated with hard bottom dominated sites with saithe being more abundant at exposed sites, compared to pollack. Goldsinny wrasse (Ctenolabrus rupestris) and corkwing wrasse (Symphodus melops) appeared to favor sugar kelp forests and red algae. Overall, the study identifies the specific sublittoral biotic habitats and substrates important to different early life stages of juvenile coastal cod and other commercially important demersal fish, providing critical information needed for identifying candidate coastal habitats for protection.

Motile fauna of sub-tidal Zostera marina meadows in England and Wales

Despite being under continued anthropogenic threat, there exists limited evidence in the United Kingdom (UK) and northern Europe for the role of sub-tidal Zostera marina meadows in providing habitat in support of economically important fauna. This is a major issue given the need to support fisheries production into the future. Understanding this at a regional and local scale is important given that their ecosystem service value can change with factors such as environmental gradients and anthropogenic impacts. In the present study, we investigate the role of sub-tidal seagrass in supporting motile fauna including juvenile fish and invertebrates of commercial value. Seagrass meadows in three locations in the UK (Porthdinllaen, Tremadog Bay and the Isle of Wight) were examined using stereo Baited Remote Underwater Video systems (BRUVs). Twenty-six taxa were recorded, ten of which were of economic importance, including an abundance of juvenile gadoids. Although the commercially important species that we found as juveniles in seagrass are not obligate seagrass users, the resources that seagrass meadows offer to these fish potentially provide significant long-term fitness benefits, possibly enhancing the whole population. All sites sampled contained economically important fauna, but there existed a significant difference between the assemblages at different locations, with the sites in the Isle of Wight containing an impoverished fauna. Sites sampled in North Wales contained double the abundance of individuals and over three times the number of species than those in the Isle of Wight. This highlights that ecosystem services such as habitat provision are not constant between sites and possibly impacted upon by site condition and locally specific environmental differences. The present study provides evidence of the importance of seagrass meadows in the UK for supporting biodiversity and the need to protect these sensitive habitats.

Is the poleward expansion by Atlantic cod and haddock threatening native polar cod, Boreogadus saida?

During a recent period of increased influx of warm Atlantic water to the western coast of Svalbard, we have observed a northward expansion of boreal Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) into areas dominated by the native polar cod (Boreogadus saida). To determine the potential impact of new ecological interactions, we studied the diet of co-occurring juvenile gadoids in fjords, open water, and sea ice around Svalbard. We also reviewed the available literature on polar cod feeding in different habitats across the Arctic to determine whether region, habitat, or fish size may influence diet. Feeding by polar cod in the pelagic zone was size dependent, with small fish primarily consuming Calanus spp. and smaller copepods, with an increasing ration of Themisto spp. at larger sizes. In benthic habitats, diets were more varied and included considerably more unidentified material and sediment. Less than 40% dietary overlap was detected among the three species when they were found together. Stable isotope analyses indicated these patterns were representative of longer-term assimilation. The low interspecific dietary overlap suggests little direct competition. Future increases in abundance and the high predation potential of the boreal taxa, however, may impact the persistence of polar cod on some Arctic shelves.

Using acoustic technology to improve the modelling of the transportation and distribution of juvenile gadoids in the Barents Sea

Abstract Johansen, G. O., Godø, O. R., Skogen, M. D., and Torkelsen, T. 2009. Using acoustic technology to improve the modelling of the transportation and distribution of juvenile gadoids in the Barents Sea. – ICES Journal of Marine Science, 66: 1048–1054. Transport of the juvenile stages of gadoids by oceanic currents in the Barents Sea is governed by interactions between the vertical positioning of the fish and the horizontal movement of the water masses. The resulting geographical distribution is important for growth and survival. There is need for observations at proper temporal and spatial scales to improve the representation of vertical distribution in models of the transport process. Stationary acoustic systems are suitable for this purpose. We use such a system to quantify the vertical dynamics of 0-group gadoids with reference to a conceptual model of the temporal variation. The vertical distribution from the conceptual model is applied within a Lagrangian, particle-tracking model. This approach performs better in predicting the geographic distribution of the 0-group during the first 10 months after hatching than a model with random, vertical distribution. The potential of stationary acoustic systems to provide high-quality vertical distributions that improve the predictive power of the transport model is demonstrated. Extensive sampling programmes based on the principles presented here can provide the observations needed to obtain more realistic recruitment–prediction models.

A method for the possible species discrimination of juvenile gadoids by broad-bandwidth backscattering spectra vs. angle of incidence

AbstractLundgren, B., and Nielsen, J. R. 2008. A method for the possible species discrimination of juvenile gadoids by broad-bandwidth backscattering spectra vs. angle of incidence. – ICES Journal of Marine Science, 65: 581–593. Measurements were made of the broad-bandwidth (80–220 kHz) acoustic backscattering from free-swimming juvenile gadoids at various orientations and positions in an acoustic beam, under controlled conditions. The experimental apparatus consisted of a stereo-video camera system, a broad-bandwidth echosounder and echo-processor system, a narrowband 120 kHz split-beam echosounder, a large tank, and a fishnet cage. The net cage was centred on the acoustic beams and was virtually transparent, both acoustically and optically. Accurate three-dimensional positions and angular orientations of individual fish were estimated from stereo-images captured synchronously when broad-bandwidth echoes were received from passing fish. Fish positions were also estimated from data collected with a synchronized split-beam echosounder. Software was developed for image analysis and modelling, including calibration, alignment of acoustic and optical-reference frames, and automatic position-fitting of fish models to manually marked fix-points on fish images. The software also performs Fourier spectrum analysis and pulse-shape analysis of broad-bandwidth echoes. Therefore, several measurement series on free-swimming juvenile gadoids were evaluated. The method and data may be used to improve the acoustic identification of fish species and sizes, and thereby improve investigations of spatial prey–predator relationships, and the accuracy and efficiency of acoustic surveys.

Recent work to improve the efficacy of square-mesh panels used in a North Sea Nephrops norvegicus directed fishery

In the continuing drive to reduce discards of juvenile gadoids (whiting, cod, haddock) in the Farn Deeps Nephrops directed fishery in the North Sea, we examine the efficacy of the square-mesh panels (SMP) which are currently fitted to the trawls in that fishery. We find that discards can be further reduced by around 40% through simple modification to these square-mesh panels. While some basic measures to improve their efficacy have been identified, we have not yet described the ‘optimal square-mesh panel’ and thus we indicate areas for further research in this field which may yield further dividends in discard reduction.

Gape morphology of cod Gadus morhua L., haddock Melanogrammus aeglefinus (L.) and whiting Merlangius merlangus (L.) through metamorphosis from larvae to juveniles in the western Irish Sea

Variations in standard length (LS), gape size (SG) and jaw length (LJ) were studied in larval and juvenile gadoids (cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus) from 4 to 70 mm. The increase in SG and LJ was not linear with respect to LS. The relationship was best described by segmented regression lines in all three species, with an inflection point at c. 10·5 mm. The SG and LJ increased more rapidly in relation to larval LS for individuals smaller than this inflection point size. The rates of increase slowed significantly post‐inflection, an effect more noticeable in SG data compared to LJ data. In each case, the inflection point fell in the intermediate period of development between the larval and juvenile stages, which could be considered as metamorphosis. Published equations that have been used to predict SG from LJ lead to the overestimation of gape. New relationships are presented, which may be used to predict SG from measurements of either LS or upper jaw length in cod, haddock and whiting.

Enhanced survival of 0-group gadoid fish under jellyfish umbrellas

Young (0-group) gadoid fish, which have been observed sheltering beneath jellyfish (Scyphozoa and Hydrozoa) umbrellas, may find refugia from predation by retreating among medusan tentacles. The survival of juvenile gadoids may therefore be improved by high abundances of medusae. Jellyfish (including Cyanea lamarckii and C. capillata) were caught in the North Sea during routine sampling for 0-group gadoids (cod Gadusmorhua, haddock Melanogrammus aeglefinus, Norway pout Trisopterus esmarkii and whiting Merlangius merlangus) between 1971 and 1986, and considerable overlaps have been shown between the spatial distributions of these fish and medusae. Here correlations are made between the abundance of medusae in the North Sea and the residual survival of 0-group fish, as quantified by the deviation in 1-group recruitment from the expected Ricker modelled estimate. Significant positive correlations between the residual survival of whiting and medusa abundance are evident for each individual Cyanea sp. and for the combined Cyanea spp. ln(maximum) abundance (all R ≥ 0.60, P < 0.01, N = 15). The abundance of jellyfish may thus be an important factor influencing the mortality of whiting in the North Sea, and as such should be considered in the development of ‘ecosystem-based’ management of whiting stocks.

The effective selectivity of a composite gear for industrial fishing: a sorting grid in combination with a square mesh window

A new gear consisting of a sorting grid in combination with a square mesh window was developed to remedy the bycatch of juvenile gadoids in the industrial fishery for Norway pout ( Trisopterus esmarki ). The purpose of square mesh window was to retain larger marketable fish otherwise sorted out by the grid. Experimental fishing with the composite gear showed improved selectivity of a commercial trawl with catch weight reductions of haddock ( Melanogrammus aeglefinus ) and whiting ( Merlangius merlangus ) of 37 and 57%. Application of these reduction percents to the historical level of industrial bycatch in the North Sea lowered on average the yearly haddock bycatch from 4.3 to 2.7% of the equivalent spawning stock biomass. For whiting the theoretical reduction was from 4.8 to 2.1%. Underlying these results is the deployment of a novel methodology to describe the effective selectivity of composite gears, where the usual descriptors of trawl selectivity, l 50% and SR, do not apply. In order to develop this methodology, fish were collected from three compartments of the gear during the experimental fishing. This experimental set up allowed us first to determine separate selectivity estimates of both the grid and the window and secondly to combine these into estimates of the effective selectivity of the composite gear.

The effective selectivity of a composite gear for industrial fishing: a sorting grid in combination with a square mesh window

A new gear consisting of a sorting grid in combination with a square mesh window was developed to remedy the bycatch of juvenile gadoids in the industrial fishery for Norway pout (Trisopterus esmarki). The purpose of square mesh window was to retain larger marketable fish otherwise sorted out by the grid. Experimental fishing with the composite gear showed improved selectivity of a commercial trawl with catch weight reductions of haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) of 37 and 57%. Application of these reduction percents to the historical level of industrial bycatch in the North Sea lowered on average the yearly haddock bycatch from 4.3 to 2.7% of the equivalent spawning stock biomass. For whiting the theoretical reduction was from 4.8 to 2.1%. Underlying these results is the deployment of a novel methodology to describe the effective selectivity of composite gears, where the usual descriptors of trawl selectivity, l50% and SR, do not apply. In order to develop this methodology, fish were collected from three compartments of the gear during the experimental fishing. This experimental set up allowed us first to determine separate selectivity estimates of both the grid and the window and secondly to combine these into estimates of the effective selectivity of the composite gear.

Small-scale distribution of juvenile gadoids in shallow inshore waters; what role does maerl play?

Small-scale distribution of juvenile gadoids in shallow inshore waters; what role does maerl play?

Small-scale distribution of juvenile gadoids in shallow inshore waters; what role does maerl play?

Abstract The indirect effects of demersal fisheries, such as habitat degradation, are currently thought to be impacting gadoid stocks. Maerl fulfils nursery area prerequisites for several invertebrate species, so its role in similar ecosystem service provision for gadoids has been addressed. Juvenile cod (Gadus morhua), saithe (Pollachius virens), and pollack (Pollachius pollachius) in shallow (&lt;7 m) inshore waters were surveyed with fykenets and scuba off western Scotland over a period of 12 months. Juvenile densities were highest from September to November, and at that time, significantly more were present during the day and associated with maerl (that lacked macroalgal cover) than with heavily vegetated rocky and gravel substrata. Juvenile cod were present throughout the year, whereas saithe appeared in July, and pollack from September to January. With its abundance of food, maerl probably has a high holding capacity for juvenile gadoids, and thus is an important part of the inshore nursery system.

Trawl hangs, baby fish, and closed areas: a win–win scenario

Abstract The frequency and geographic distribution of trawlnet hangs from a fishery-independent survey are evaluated. The hangs data were plotted on a substratum map to confirm that many, but not all, were naturally occurring, high relief substrata. The data were also coupled with the occurrence of juvenile cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) to assess the degree of association between juvenile gadoids and high relief substrata. The average minimal distance from a fish occurrence to a hang ranged from 8.1 to 12.0 km (4.4–6.5 nautical miles), well within the reported daily range of movement for these fish. A similar pattern was detected for the sea raven (Hemitripterus americanus), a predator of juvenile gadoids, confirming the location of these microhabitat foodwebs. On average, closing an area 3.7 km (2 nautical miles) around a hang will enclose 17–30% of the populations of these juvenile fish; a wider buffer (18–28 km; 10–15 nautical miles) will close a linearly increasing portion of the populations. Additionally, closing areas surrounding the hangs, particularly regions of high hang density, will help to minimize losses of or damage to fishing gear. We propose a win–win scenario by establishing or evaluating closed areas in regions with high concentrations of known hangs. This approach is widely applicable for many marine ecosystems and may help to achieve simultaneous conservation and resource management goals, whereby one can both protect pre-recruit fish and enhance the effectiveness of a fishery.

Episodic Mass Invasions of Juvenile Gadoids into the Wadden Sea and their Consequences for the Population Dynamics of Brown Shrimp (Crangon crangon)

Abstract. The brown shrimp Crangon crungon is considered to be a key species in the coastal waters of the North Sea and in particular in the Wadden Sea, since it occurs in masses and acts as both a highly efficient predator and important prey. The Wadden Sea is the nursery ground for brown shrimp. In 1990, a mass invasion of O‐group whiting (Merlangius merlangus) reduced Crangon numbers to almost zero. Similar events, which had previously been reported in the 19th century, were observed in 1959 (whiting), 1970 (cod, Gadus morhua), and 1983 (cod and whiting).The 1990 invasion and the decline of the shrimp stock are documented on the basis of surveys that are compared with the years preceding and following. Consequences for the food web in the Wadden Sea and the reasons for the complete recovery of the shrimp stock in each case within one year after its collapse are discussed with regard to the Crangon crungon behaviour and reproductive potential. Sampling error and the impact of fisheries are also considered.

The forth estuary: a nursery and overwintering area for North Sea fishes

The fish community of the Forth estuary, Scotland, has several components — estuarine resident species, diadromus migratory species, marine and freshwater adventitious species, marine juvenile migrants using the area as a nursery, and adults of marine species with seasonal migrations. The population changes during the period 1981–1988 in six species representative of the last two categories are described here. The species are the juvenile gadoids, whiting, Merlangius merlangus (L.) and cod, Gadus morhua L., juvenile flatfish, plaice, Pleuronectes platessa L. and common dab, Limanda limanda (L.), and the adult clupeids, sprat, Sprattus sprattus (L.) and herring, Clupea harengus L.