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

Abstract

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.

Age-structure, harvesting and climate effects on population growth of Arcto-boreal fish stocks

“…Note that following the methods used, if the elasticity to recruitment increases, the elasticity to survival decreases, and vice versa. However, transient elasticity does not tell us how much and Gulf of Alaska (GOA) pollock Theragra chalcogramma stocks, North Sea (NS) cod stock, northern Atlantic (NA), southern Atlantic (SA) and Balearic Islands (BI) hake Merluccius merluccius stocks are from Durant et al (2013) the population is actually changing, but it helps us to understand the processes underlying these changes. In our study, the pre-eminent factor is the negative association between the generation time μ and the transient elasticity to recruitment, confirming a previous study conducted on other data for gadoids only ).…”

“…In order to evaluate how recruitment affects spawning stock biomass and population numbers, we performed an analysis of transient elasticity to recruitment variation (hereafter called elasticity e; e being the contribution of recruitment or survival to population growth changes) following the principles defined by Caswell (2007) and methods described by Durant et al (2013). We used numerical methods based on simulating the population from year t to year t + t g .…”

Harvested fish stocks in a changing environment

“…Durant et al (2013) showed that the contribution of recruitment to population growth was affected by environmental conditions and by the age-structure of the population. However, contrary to what was expected, Ottersen et al (2013) found little or no general decrease in mean age in the spawning stock.…”

“…Juvenation may increase a populations' ability to directly respond to environmental fluctuations (Rouyer et al 2011), emphasizing the importance of the interaction between fisheries, environment, and recruitment and population growth that produces complex synergic effects on the population dynamics of marine species. Durant et al (2013, this Theme Section) explored how external forcing (climate and fishing) and recruitment modulate the population dynamics of fish within different environments. They calculated the realized yearly population growth rate of 7 gadoid stocks across different ecosystems.…”

“…By comparing the corresponding sensitivity of population growth to recruitment change across the 7 stocks, Durant et al (2013) showed that reduction in generation time triggered an increase in the relative contribution of recruitment to population growth, while the increase of fishing mortality -suggested to truncate the age structure of the stocks -had a weaker effect. The findings of Durant et al (2013) emphasise how the elasticity of the population growth to recruitment change can be affected by a long-term change in age structure of the population, maybe due to fishing (e.g. Rouyer et al 2011, but see Ottersen et al 2013), but also directly by the ongoing fishing intensity.…”