Oceanography and marine biogeochemistry

Climatic changes are disrupting otherwise tight trophic interactions between predator and prey. Most of the earlier studies have primarily focused on the temporal dimension of the relationship in the framework of the match-mismatch hypothesis. This hypothesis predicts that predator's recruitment will be high if the peak of the prey availability temporally matches the most energy-demanding period of the predators breeding phenology. However, the match-mismatch hypothesis ignores the level of food abundance while this can compensate small mismatches. Using a novel time-series model explicitly quantifying both the timing and the abundance component for trophic relationships, we here show that timing and abundance of food affect recruitment differently in a marine (cod/zooplankton), a marine-terrestrial (puffin/herring) and a terrestrial (sheep/vegetation) ecosystem. The quantification of the combined effect of abundance and timing of prey on predator dynamics enables us to come closer to the mechanisms by which environment variability may affect ecological systems. Ecology Letters, (2005) 8: 952-958

Atlantic cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto-Norwegian (or North-east Arctic) cod stock in the Barents Sea is now the largest stock of Atlantic cod. Recruitment to this stock has varied extensively during the last 60 yr. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for this variability, higher temperatures being favourable for survival throughout the critical early life stages. Our studies of time series present compelling evidence for a strengthening of the climate-cod recruitment link during the last decades. We suggest this is an effect of the age and length composition of the spawning stock having changed distinctly. The age of the average spawner has decreased by more than 3 yr from between 10 and 11 in the late 1940s to 7-8 in the 1990s, average length from just above 90 cm to around 80 cm. The number of age classes contributing to the spawning stock has also decreased, while the number of length groups present increased slightly. Significant decrease in age of spawners has frequently been described for other heavily fished stocks worldwide. We therefore find it likely that the proposed mechanism of increased influence of climate on recruitment through changes in the spawning stock age and size composition is of a general nature and might be found in other systems.

Much interest has recently been devoted to reconstructing the dynamic structure of ecological systems on the basis of time-series data. Using 10 years of monthly data on phyto-and zooplankton abundance from the Bay of Biscay (coastal to shelf-break sites), we demonstrate that the interaction between these two plankton components is approximately linear, whereas the effects of environmental factors (nutrients, temperature, upwelling and photoperiod) on these two plankton population growth rates are nonlinear. With the inclusion of the environmental factors, the main observed seasonal and inter-annual dynamic patterns within the studied plankton assemblage also indicate the prevalence of bottom-up regulatory control.

Atlantic cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto-Norwegian (or North-east Arctic) cod stock in the Barents Sea is now the largest stock of Atlantic cod. Recruitment to this stock has varied extensively during the last 60 yr. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for this variability, higher temperatures being favourable for survival throughout the critical early life stages. Our studies of time series present compelling evidence for a strengthening of the climate–cod recruitment link during the last decades. We suggest this is an effect of the age and length composition of the spawning stock having changed distinctly. The age of the average spawner has decreased by more than 3 yr from between 10 and 11 in the late 1940s to 7–8 in the 1990s, average length from just above 90 cm to around 80 cm. The number of age classes contributing to the spawning stock has also decreased, while the number of length groups present increased slightly. Significant decrease in age of spawners has frequently been described for other heavily fished stocks worldwide. We therefore find it likely that the proposed mechanism of increased influence of climate on recruitment through changes in the spawning stock age and size composition is of a general nature and might be found in other systems.

Planktivorous capelin is a key species in the Barents Sea, being of great importance in the exploitation of plankton production in this subarctic region. However, in years with a successful reproduction of the Norwegian spring-spawning herring, large amounts of herring larvae drift into the Barents Sea, where they stay for 2 to 3 yr. The 1 to 2 yr old herring has a pronounced impact on capelin, eating large amounts of capelin larvae. The main fish predator of the Barents Sea, the Arcto-Norwegian cod, also consumes large amounts of post-larval capelin. In this study, we show how temperature and the North Atlantic Oscillation (NAO) indirectly influence the population dynamics of capelin by influencing the reproduction of herring and cod. After 1980, when the herring spawning stock had recovered after its collapse in 1969, we found that temperature strongly negatively influences capelin cohorts 2 yr before spawning. Capelin cohorts which have spawned 2 yr after a warm year tend to experience high predation from both young herring as larvae and from 3 to 6 yr old cod during the rest of their life. Other analyses confirm that present sea temperatures or previous NAO conditions have strong positive effects on the abundance of 0-group cod and herring. Thus, the climatic regime of the region ultimately determines the balance between capelin and herring, which in turn has pronounced consequences for the species composition and energy flow of the entire ecosystem.

The following supplement accompanies the article . Relationship between environmental and biological variables as found in the seabird literature (a selection of seabird literature, 127 articles). Environmental variables include: weather = weather packages (ENSO, NAO…); oceanography = fronts, eddies, currents, etc.; SST = sea surface temperature; prey = prey abundance, location, etc.; integrate index = index issue from statistical calculation (e.g. principal component). Biological variables include: behaviour = behaviour (outside foraging), phenology, etc.; breeding success = breeding success sensus largo that encompasses the fledging success (no. chicks that fledge per no. chicks in nest), survival through the first year, etc.; condition = body condition (mass, reserves, etc.); diet = diet, food deliveries etc; foraging = foraging activity, transit, exploration, diving, etc.; growth = chick growth (mass, linear, development, etc.); population = population parameters, etc.; survival = adult survival. Last two rows of the table: references from telemetry (GPS, satellite, loggers…) studies and reviews Environmental variable Biological variable Source Fishing Breeding success

Two factors determine whether pollution is likely to affect a population indirectly through loss of prey: firstly, the sensitivity of the prey to the pollutants, and secondly, the sensitivity of the predator population to loss of prey at the given life stage. We here apply a statistical recruitment model for Northeast Arctic cod to evaluate the sensitivity of cod cohorts to loss of zooplankton prey, for example following an oil spill. The calculations show that cod cohorts are highly sensitive to possible zooplankton biomass reductions in the distribution area of the cod larvae, and point to a need for more knowledge about oileffects on zooplankton. Our study illustrates how knowledge about population dynamics may guide which indirect effects to consider in environmental impact studies.

We analyse the population dynamics of Northeast Arctic cod (Gadus morhua) by applying statistical population models to 22 years of research survey data on abundance, length, and maturation. The models for abundance and individual length are selected using Akaike's information criterion (AIC C ). Survival of 2-to 4-year-old cod was found to be negatively related to the abundance of cod relative to its preferred prey, capelin (Mallotus villosus), likely the result of increased cannibalism from older cod when there is a lack of capelin. Growth up to age 4 decreases with abundance and increases with sea temperature and the North Atlantic Oscillation index, whereas growth from age 4 to age 8 decreases with increasing cod-capelin ratio. The statistical models were combined in a dynamic age-structured model, coupling the dynamics of abundance and body length (reproduction depends on body length, and length growth is influenced by abundance). Simulations using this model are able to recreate the main abundance patterns of each age group for 1982-2002. The model's ability to predict changes in the abundance of spawners appeared to be most limited by our ability to predict survival of 3-to 6-year-old immature cod.

The vast majority of the world's fisheries are typically managed within a single-species perspective, ignoring the dynamic feedback mechanisms generated by the ecological web of which they are a part. Here we show that the dynamics of the Barents Sea capelin (Mallotus villosus), the world's largest stock of this species, is strongly influenced by both within-system ecological feedback mechanisms and the impact of harvesting. Both overexploitation and predation by herring (Clupea harengus) can cause the population to collapse, whereas predation by cod (Gadus morhua) is demonstrated a delay in the stock's recovery after a collapse. Such collapses, which have occurred twice in 20 years, affect the entire Barents Sea ecosystem, a region that for ages has provided food for all of Europe.

The vast majority of the world's fisheries are typically managed within a single-species perspective, ignoring the dynamic feedback mechanisms generated by the ecological web of which they are a part. Here we show that the dynamics of the Barents Sea capelin (Mallotus villosus), the world's largest stock of this species, is strongly influenced by both within-system ecological feedback mechanisms and the impact of harvesting. Both overexploitation and predation by herring (Clupea harengus) can cause the population to collapse, whereas predation by cod (Gadus morhua) is demonstrated a delay in the stock's recovery after a collapse. Such collapses, which have occurred twice in 20 years, affect the entire Barents Sea ecosystem, a region that for ages has provided food for all of Europe. Mallotus villosus | cod | herring | harvesting | ratio dependence

Climate influences a population through a variety of processes, including reproduction, growth, migration patterns and phenology. Climate may operate either directly through metabolic and reproductive processes or indirectly through prey, predators, and competitors. One mechanism that may be particularly important, and which is the focus of this review, is the role of climate in affecting the reproductive success of a predator through its effect on the relative timing of food requirement and food availability during early life stages. This principle -the match or mismatch of predators' requirement with resource availability -originated in the marine literature, where it initially referred to how growth and survival of fish larvae (predator) depends on this production being synchronous with that of their main food items, i.e. early stage zooplankton (prey). Here we review how the match/mismatch hypothesis (MMH) is used to describe climate effects on ecological patterns and processes in both marine and terrestrial systems. In addition to studying match/mismatch sensu stricto, we expand on it to include effects of overall production level and the spatial aspect. Possible impacts of climate change on match/mismatch are examined in the context of one of the most apparent effects of global warming: an advancement of spring phenology. As a consequence of different species reacting dissimilarly, even minor changes in climate may invoke non-linear responses unbalancing established patterns of synchrony. All components of a food chain cannot be expected to shift their phenology at the same rate, and thus are unlikely to remain synchronous.

A macroecological view of the ecosystem offers the possibility to integrate information at large spatial and temporal scales over a variety of complex ecological systems. Marine macroecology can be regarded as a new research agenda aiming to develop new models which can explain the emergent structures and dynamics of complex ecological systems in terms of basic physical and biological principles (Brown 1999). Macroecology theory as a way to describe the emergent properties in terrestrial systems has received relatively little attention in marine ecology. This Theme Section is a collection of articles that will discuss the importance of macroecological and complexity theory, in a very broad context, to untangling patterns that underlie the relationships between species abundance and other biotic and abiotic factors linking organismal biology, population dynamics, community ecology, food web structure, biodiversity, and behavioral ecology, to ecosystem structure and function. This macroecological view of different processes underlying the dynamics of marine ecosystems extends the general theory of macroecology and allometric scaling, developed mainly for terrestrial systems (Brown 1995, West et al. 1997, to a marine context as recently proposed by Li (2002). Ultimately we need to understand by first principles, from organism organization to ecosystem organization (Reynolds 2001), the basic common ecological rules that generate the variability and patterns that we observe across scales. LITERATURE CITED Brown JH (1995) Macroecology. Chicago University Press, Chicago Brown JH (1999) Macroecology: progress and prospect. Oikos

Proper management of ecosystems requires an understanding of both the species interactions as well as the effect of climate variation. However, a common problem is that the available time-series are of different lengths. Here, we present a general approach for studying the dynamic structure of such interactions. Specifically, we analyse the recruitment of the world's largest cod stock, the Northeast Arctic cod. Studies based on data starting in the 1970-1980s indicate that this stock is affected by temperature through a variety of pathways. However, the value of such studies is somewhat limited by the fact that they are based on a quite specific ecological and climatic situation. Recently, this stock has consisted of fairly young fish and the spawning stock has consisted of relatively few age groups. In this study, we develop a model for the effect of capelin (the cod's main prey) and herring on cod recruitment since 1973. Based on this model, we analyse data on cod, herring and temperature going back to 1921 and find that food-web effects explain a significant part of the cod recruitment variation back to around 1950.

Interactions within and between species complicate quantification of climate effects, by causing indirect, often delayed, effects of climate fluctuations and compensation of mortality. Here we identify direct and indirect climate effects by analysing unique Russian time-series data from the Norwegian Sea -Barents Sea ecosystem on the first life stages of cod, capelin, herring and haddock, their predators, competitors and zooplanktonic prey. By analysing growth and survival from one life stage to the next (eggs -larvaejuveniles -recruits), we find evidence for both bottom-up, direct and top-down effects of climate. Ambient zooplankton biomass predicts survival of all species, whereas ambient temperature mainly affects survival through effects on growth. In warm years, all species experienced improved growth and feeding conditions. Cohorts born following a warm year will, however, experience increased predation and competition because of increased densities of subadult cod and herring, leading to delayed climate effects. While climate thus affects early growth and survival through several mechanisms, only some of the identified mechanisms were found to be significant predictors of population growth. In particular, our findings exemplify that climate impacts are barely propagated to later life stages when density dependence is strong.

Timing of breeding is a key factor determining the reproductive success in bird populations and known to be affected by climate fluctuations. We investigated the longterm relationship between climate and hatching date within a population of Atlantic puffin Fratercula arctica at Røst in the Norwegian Sea. The timing of puffin breeding was found to be influenced by the North Atlantic Oscillation winter index (NAO). We isolated two temporal regimes, one where NAO had a significant effect on hatching date (1978-1986 and 1995-2002) and one where these variables were independent (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994). Hatching date could be modelled using, in addition to NAO, hatching date and food abundance in the preceding breeding season (possibly proxies of parental effort). The models remained significant for regime 1 but not for regime 2. NAO differed between the two regimes suggesting that the shifts were induced by climate change, possibly via its effect on the availability of prey in the preceding year. The novelty of our study is the identification of temporal regimes in the effects of climate within one population.

The North-East Arctic cod (Gadus morhua) fishery, as most other commercial fisheries, is wasting the larger part of its potential. Examining a detailed multi-cohort, multi-gear bioeconomic model, we show that the cause is rather a too small mesh size than excessive effort. Although internationally and nationally managed by quota allocations and regulations, the current exploitation pattern implies that essentially the wrong fish are targeted. Catching older and heavier fish could double the fishery’s Net-Present-Value. This increases harvested biomass while it reduces the number of fish removed from the ocean, highlighting the importance of age- and gear-specific modeling. Moreover, optimal harvesting policies would also lead to a much more robust and abundant cod stock.