In marine ecosystems, maximum sustainable yield considerations are affected by any substantial changes that occur in the top and bottom compartments of the food-web. This study explores how the ...southern North Sea's fisheries may need to adjust their fishing efforts to maintain optimum yields of sole, plaice, cod and brown shrimps under increased marine mammal populations and a reduced primary productivity. We constructed plausible scenarios of ongoing food-web changes using the results of Bayesian age-structured population models to estimate carrying capacities of harbour porpoises (Phocoena phocoena) and grey seals (Halichoerus grypus). Losses in primary productivity were predicted by lower trophic level ecosystem models. These scenarios were implemented in a food-web model of the southern North Sea. For each scenario, we sought mixed-fleet fishing efforts that would deliver maximum yields of sole, plaice, cod and brown shrimp combined. We also did so for a baseline run with unaltered mammal and primary production, and compared the differences in optimal fishing strategies, predicted yields, and states of the stocks between the scenarios. We found stocks and yields to be far more sensitive to changes in primary productivity than to increased marine mammal predation. The latter predominantly impacted cod, and even benefitted brown shrimps compared to the baseline run. Under 30% reduced primary productivity, fishing efforts had to be reduced by 50% to still provide maximum yields, whereas the marine mammal scenario induced no need to adjust the fishing regime. This draws attention to the potential gains of incorporating bottom-up processes into long-term management considerations, while marine mammal predation may be less of a concern, in particular for flatfish fisheries in the North Sea, and may even benefit shrimp trawlers because of reduced predation on shrimp from fish predators.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further ...validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC,
Gadus morhua
) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.
Oxygen uptake (MO2; mgO2 fish-1h-1) of fish groups was measured at temperatures between 10-19°C in an intermittent-flow respirometer to quantify the metabolic costs of spontaneous swimming patterns ...in the small clupeid Sprattus sprattus. Movements of individual fish within the school were tracked automatically during respirometry. Oxygen uptake was then related to mean swimming speeds and the number of sharp turns (>90°), which are common behavioural elements of spontaneous swimming in clupeid fish. Different possible model formulations for describing the relationship between respiration and swimming patterns were compared via the AIC. The final model revealed that costs for sharp turns at a frequency of 1 s-1 doubled the metabolic costs compared to those with zero turns but with likewise a moderate swimming speed of 0.28 body length -1. The cost for swimming doubled if the swimming speed was doubled from 0.28 to 0.56 BLs-1 but increased by a factor of 4.5 if tripled to 0.84 BLs-1. Costs for transport were minimal at a speed of 0.4 body lengths s-1 at all temperatures. New basic input parameters to estimate energy losses during spontaneous movements, which occur typically during foraging in this small pelagic fish, are provided.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Growth rates are of fundamental interest for studying population dynamics of species when no age information is available. In these cases in situ growth proxies must be identified and validated, and ...here we tested whether RNA-DNA ratio (RD) and dry weight condition (DWC) can act as such proxies for the shrimp Crangon crangon. Growth rates (mm d–1) were determined for male and female shrimps (20 to 67 mm) reared at five different temperatures (5°C to 25°C) and caught in two different months (May, July). Due to the potential interactive effects of the factors sex, size, temperature and time, different statistical analysis were applied to test for linear interactions (factorial analysis of mixed data, FAMDs), nonlinear additive effects (generalized additive models, GAMs) and nonlinear interactions (nonlinear regression combined with GAMs). The FAMD indicated positive correlations between length, month and growth rate. A positive correlation between DWC and sex and between RD and temperature was also indicated. RD was not significant in the GAM but a nonlinear link between length, temperature and growth was found. Finally, an extended Bertalanffy growth model was fitted to sex-specific growth rates and in a second step a GAM was fitted to the differences (residuals) between observed and predicted growth. RD again had no significant explanatory power for growth and, although it is a commonly used growth proxy, we concluded that it is not a suitable index for evaluating length growth of C. crangon.
•We parameterized an ecosystem model of the southern North Sea.•We sought for fishing effort regimes that would provide multispecies MSYs.•It was impossible to achieve good yields of all scope ...species simultaneously.•Aligning MSY and GES requires trade-offs between conservation and fishing goals.
The southern North Sea is the stage of conflicting objectives of beam trawlers targeting flatfish; of shrimp trawlers fishing for brown shrimp with bycatch of juvenile flatfish; and of demersal trawlers, the main fishery on cod, a key predator of shrimp and other groups. To expose trade-offs between the fleets’ objectives and to explore what a possible variant of a multispecies maximum sustainable yield (MSY) could look like, we parameterized an ecosystem model and subjected it to a range of different fishing effort levels of the three fleets. Long-term projections highlighted multiple fishing regimes that lead to catches of at least 30% of all focal stocks' single species MSYs at the same time. Trade-offs between the yields of shrimp fishers and demersal trawlers made higher simultaneous yields impossible. Besides optimizing multispecies catches, we identified effort regimes that satisfied a set of descriptors of good environmental status (GES). We found that GES can only be obtained through low efforts of beam and demersal trawlers, which cannot be aligned with our multispecies MSY variant without accepting trade-offs in fishing yields and/or conservation goals.
•First time bio-physical empirical indication of OWF effects on the pelagic ecosystem.•High speed, high-resolution remotely operated vehicle (TRIAXUS ROTV) transects.•We provide empirical indication ...for increased vertical mixing within the OWFs.•There was doming of the thermocline and vertical nutrient transport within the OWFs.•Submesoscale chlorophyll-a pillars occurred at regular intervals.
A recent increase in the construction of Offshore Wind Farms (OWFs) has initiated numerous environmental impact assessments and monitoring programs. These focus on sea mammals, seabirds, benthos or demersal fish, but generally ignore any potential effects OWFs may have on the pelagic ecosystem. The only work on the latter has been through modelling analyses, which predict localised impacts like enhanced vertical mixing leading to a decrease in seasonal stratification, as well as shelf-wide changes of tidal amplitudes. Here we provide for the first-time empirical bio-physical data from an OWF. The data were obtained by towing a remotely operated vehicle (TRIAXUS ROTV) through two non-operating OWFs in the summer stratified North Sea. The undulating TRIAXUS transects provided high-resolution CTD data accompanied by oxygen and chlorophyll-a measurements. We provide empirical indication that vertical mixing is increased within the OWFs, leading to a doming of the thermocline and a subsequent transport of nutrients into the surface mixed layer (SML). Nutrients were taken up rapidly because underwater photosynthetically active radiation (PAR) enabled net primary production in the entire water column, especially within submesoscale chlorophyll-a pillars that were observed at regular intervals within the OWF regions. Video Plankton Recorder (VPR) images revealed distinct meroplankton distribution patterns in a copepod-dominated plankton community. Hydroacoustic records did not show any OWF effects on the distribution of pelagic fish. The results of a pre-OWF survey show however, that it is difficult to fully separate theanthropogenic impacts from the natural variability.
During male–male competition, evolution can favor alternative reproductive tactics. This often results in a dominant morph that holds a resource, such as a nest for egg laying, which competes with a ...smaller sneaker morph that reproduces by stealing fertilizations. The salinity environment can influence male growth rates, for example, via osmoregulatory costs, which in turn may influence the use of sneaker tactics for small males competing for mating opportunities. Salinity can also affect sperm directly; however, little is known of how salinity influences sneaker tactics through sperm performance. We sampled males of the invasive round goby (Neogobius melanostomus) from two environments, a freshwater river and a brackish estuary. This fish has two male morphs: nest‐holding dark males and non‐nest‐holding light males. We examined the role of water salinity of 0, 8, and 16 on sperm performance and found that for estuarine males, a salinity of 0 reduced sperm velocity compared to a salinity of 8 and 16. Riverine males had low velocity in all salinities. Sperm viability also decreased by over 30% in 0 salinity, compared to 8 and 16, for fish from both environments. Gobies produce ejaculate contents in specialized glands that could in theory shield sperm in an adverse environment. However, gland contents did not improve sperm performance in our tests. Body mass and age estimates indicate that riverine males invested more in somatic growth compared to estuarine males. Estuarine light morph males had a high enough gonadosomatic index to indicate sneaker tactics. We propose that when sperm performance is low, such as for the riverine males, sneaker tactics are ineffective and will be selected against or phenotypically suppressed. Instead, we interpret the increased investment in somatic growth found in riverine males as a life‐history decision that is advantageous when defending a nest in the next reproductive season.
For a male fish too small to defend mating resources such as a female or her eggs, siring offspring can be achieved by sneaking into another male's nest and ejaculating there. In the invasive round goby, this alternative tactic requires sperm to function long enough to fertilize the eggs. This fish has spread into a range of salinities, but for some populations, fresh water decreases sperm velocity and lifespan, and severely limits the use of these alternative reproductive tactics to the point where they may disappear from use.
•Predicted codend selectivities were applied in a life-cycle yield-per-recruit model.•An increase in commonly used mesh size would heighten landings and biomass.•Mesh size of the past induced growth ...overfishing due to a gradual decrease in M.•Changing stock size structure with larger meshes would raise spring landings.•Density dependent effects of stock biomass on population growth are discussed.
Small mesh codends applied in the brown shrimp (Crangon crangon L.) fishery in the North Sea cause large bycatch of fish, invertebrates and undersized shrimp. We investigated the most obvious and enforceable mitigation measure: the increase of mesh size beyond 20 mm, which was the medium mesh size of the fleet before 2016. The predicted selectivity from different codend designs was applied in a yield-per-recruit model able to reproduce the seasonal pattern of landings and biomass by sex, size, and age. The model enables assessment and quantifying population responses to possible management actions, also considering other influential variables such as natural mortality, which vastly decreased during the 1990s to the contemporary low level due to the decrease of cod in the Southern North Sea. Model results show that the selectivity of 20 mm codends leads to the poorest fishery and population indexes. Any increase in mesh size within the range considered and in the present low-predator situation would lead to a larger shrimp stock and higher catches. Increasing mesh size reduces catches of undersized and small commercial-sized shrimps entering the fishery mainly in summer time, thus leading to surplus catches in autumn and spring. Model results reveal that an increase of mesh size diminishes growth overfishing and allows the formation of a larger stock that may better cope a fluctuating natural mortality and the steadily upwards creeping fishing intensity.
Understanding individual growth in commercially exploited fish populations is key to successful stock assessment and informed ecosystem‐based fisheries management. Traditionally, growth rates in ...marine fish are estimated using otolith age‐readings in combination with age‐length relationships from field samples, or tag‐recapture field experiments. However, for some species, otolith‐based approaches have been proven unreliable and tag‐recapture experiments suffer from high working effort and costs as well as low recapture rates. An important alternative approach for estimating fish growth is represented by bioenergetic modelling which in addition to pure growth estimation can provide valuable insights into the processes leading to temporal growth changes resulting from environmental and related behavioural changes. We here developed an individual‐based bioenergetic model for Western Baltic cod (Gadus morhua), traditionally a commercially important fish species that however collapsed recently and likely suffers from climate change effects. Western Baltic cod is an ideal case study for bioenergetic modelling because of recently gained in‐situ process knowledge on spatial distribution and feeding behaviour based on highly resolved data on stomachs and fish distribution. Additionally, physiological processes such as gastric evacuation, consumption, net‐conversion efficiency and metabolic rates have been well studied for cod in laboratory experiments. Our model reliably reproduced seasonal growth patterns observed in the field. Importantly, our bioenergetic modelling approach implementing depth‐use patterns and food intake allowed us to explain the potentially detrimental effect summer heat periods have on the growth of Western Baltic cod that likely will increasingly occur in the future. Hence, our model simulations highlighted a potential mechanism on how warming due to climate change affects the growth of a key species that may apply for similar environments elsewhere.
A bioenergetic model, which includes in‐situ knowledge of distribution and food intake, allows tracing (seasonal) growth patterns of cod. The model shows that summer heat periods can lead to negative energy turnover, ultimately resulting in reduced growth and potentially in a reduced reproductive potential. A detrimental effect that may occur more often in the future in the course of ongoing climate change.
The brown shrimp (Crangon crangon) fishery is of great socio-economic importance to coastal communities on the North Sea. The fishery is exploited by beam trawlers often using codends with very small ...mesh sizes, leading to concerns about catch rates of undersized shrimp. However, little information is available on codend size selection, making it difficult to provide scientifically based advice on alternative codend designs. Therefore, this study establishes a predictive framework for codend size selection of brown shrimp, based on a large selectivity dataset from 33 different codend designs tested during four experimental fishing cruises, during which more than 350,000 brown shrimp were length measured. Predictions by the framework confirm concerns about the exploitation pattern in the fishery, because the retention probability of undersized shrimp reaches 95% with the currently applied designs. The framework predictions allow the exploration of obtainable exploitation patterns depending on codend design. For example, increasing codend mesh size to 25-29 mm would reduce the retention rate of undersized shrimp to a maximum of 50%, depending on codend mesh type.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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