Theory, consequences and evidence of eroding population spatial structure in harvested marine fishes: a review

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Population growth is affected by several factors such as climate, species interaction and harvesting pressure. However, additional complexity can arise if fishing increases the sensitivity to environmental variability. To predict the effects of fisheries and climate on marine populations, there is a need for improved understanding of how they affect key ecological processes such as population growth. In this study, we used a comparative approach investigating commercially fished species across different ecosystems: the Norwegian Sea−Barents Sea (Northeast Arctic cod), the North Sea (North Sea cod), the Atlantic Ocean (European hake), the Mediterranean Sea (European hake), and the Gulf of Alaska and Bering Sea (walleye pollock). Our objective was to compare the effects of commercial fisheries, age structure and environmental variability on population growth rate. We show that although all stocks experienced a decline in abundance, only 3 of them showed a concomitant decreasing trend in generation time (South Atlantic hake, North Atlantic hake and Northeast Arctic cod), suggesting a fishing-induced erosion in their age structure. Intra-specific analysis shows that changes in generation time triggered an increase in the relative contribution of recruitment to population growth. Furthermore, the contribution from recruitment to population growth changes due to large-scale climate indices or regional-scale environmental covariates, such as sea temperature. This study illustrates how and where the interaction between large-scale ecological patterns and regional/short-scale processes are important for designing management regulations.

Section: Effects Of the Environment And Geographic Location Of The Stmentioning

confidence: 94%

Population growth across heterogeneous environments: effects of harvesting and age structure

Durant

Hidalgo²,

Rouyer³

et al. 2013

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“…Further, at low population size, a stock may lose spatial and/or genetic diversity through selection of certain population sub-units within metapopulations, which may reduce the buffering capacity of the population in response to environmental fluctuations (Hsieh et al 2010, Planque et al 2010, Ciannelli et al 2013). …”

Section: H7: the Temperature-recruitment Relationship Strengthens Witmentioning

confidence: 99%

Temporal shifts in recruitment dynamics of North Atlantic fish stocks: effects of spawning stock and temperature

Ottersen

Stige²,

Durant³

et al. 2013

Effects of variation in spawning stock and sea temperature on long-term temporal patterns in recruitment dynamics of 38 commercially harvested fish stocks in the northern North Atlantic were studied. Different statistical models were explored within a Ricker stock-recruitment framework. This includes, in order of complexity, adding a linear temperature term, a nonlinear (smooth) temperature effect, and non-stationarities (trends in intercept or in temperature effect) and finally allowing for a stepwise change (a threshold). The different models were compared in a uniform approach using Akaike's information criterion corrected for small sample size as the model selection criterion. The relationship between recruitment, spawning stock biomass and temperature varied over time. The most frequent alteration in the non-stationary linear models was, for 14 stocks, in the intercept in recruitment success, suggesting a change in pre-recruit mortality over time. Threshold models performed better than the best linear or nonlinear stationary models for 27 of the stocks, suggesting that abrupt changes (maybe even regime shifts) are common. For half of the stocks studied, the temperature effect was statistically significant when added to the model of the relationship between recruitment success and spawning stock biomass. This includes all 6 of the herring stocks studied, with a positive effect for cold-water stocks and negative effect for stocks in the more temperate southern areas. For the 4 plaice stocks analysed, all located towards the centre of the overall distribution range of plaice, a tendency toward recruitment being favoured by lower temperatures was found.

“…Population spatial structure emerges from a set of processes determining the connectivity and isolation of population subunits over space and time (reviewed by Ciannelli et al 2013). Dispersal of individuals over space will tend to erode genetic structure and synchronize population dynamics, whereas natal homing and small home-ranges will promote genetic differentiation, local adaptation, and demographic independence among subpopulations.…”

Section: Introductionmentioning

confidence: 99%

Habitat effects on population connectivity in a coastal seascape

Rogers¹,

Olsen²,

Knutsen³

et al. 2014

Knowing how population connectivity varies across heterogeneous habitats can provide insight into the mechanisms underlying population structuring, and ultimately help to inform conservation and management actions. We studied whether the spatial extent of connectivity in coastal Atlantic cod Gadus morhua varied with coastal topography, hypothesizing that connectivity during all life stages would be greater among open, exposed habitats compared to sheltered, fjord-like habitats. We tested this hypothesis using (1) an extensive mark-recapture data set to analyze movements of age 1+ cod along a gradient from inshore (sheltered) to offshore (exposed) habitats, and (2) an analysis of demographic dependence (recruitment synchrony) among exposed versus sheltered areas using a 93 yr spatially explicit time series of juvenile (age-0) abundance. We found that habitat did have a significant effect on both the scale of recruitment synchrony and movement distances, both of which were greater in exposed regions relative to sheltered. However, even in open habitats, tagged cod were highly sedentary, and connectivity among regions due to movement of age 1+ individuals was exceptionally low. Thus, despite likely differences among habitats in dispersal of egg and larval stages, older cod were highly site-attached regardless of habitat. These results highlight how physical habitat interacts with behavior to determine the scale of population structuring in marine fishes.