Fish live at different depths. Some species are pelagic and live in the upper sunlit ocean. Others are feeding on benthos and are abundant in areas where life at the seafloor is plentiful. In between these groups are the mesopelagic fish that live in the twilight zone during the day and migrate up in the water during the night. This vertical variation in fish has been recognized as important for predicting fisheries production under climate change and for quantifying the role of fish in carbon and nutrient cycling. In this study, we show how a trait-based approach can be used to obtain structurally different food web types that predictably vary with seafloor depth and ocean productivity across ocean biomes.

The trait-based configuration used diversified possible fish guilds, food web structures and energy flows compared to the more commonly used size-based models. This diversification enables practical applications that have hitherto been elusive. For example, fisheries assessments of mesopelagic fish in the open seas and the quantification of carbon export to depth by fish.

Read the paper here:
van Denderen PD, CM Petrik, CA Stock & KH Andersen (2021) Emergent global biogeography of marine fish food webs Global Ecology and Biogeography http://doi.org/10.1111/geb.13348

Large-scale climate change projections of fish generally assume that warming waters enhance fish growth. If correct, tropical fish should grow much faster than temperate and boreal fish. But do they?

Here we follow up on that question by examining how temperature influences growth of marine fish in nature from polar to tropical environments. Our results show that fish do grow faster in warmer waters, but much less than is typically predicted by the Metabolic Theory of Ecology and used in modelling studies that predict climate change impacts.   Importantly, some fish guilds, such as large demersal fish, are almost unaffected in average growth over a 30 °C gradient, whereas other fish guilds, such as small pelagic fish and elasmobranch, increase much stronger. These results highlight that 1) the importance of temperature is overestimated in most large-scale climate change projections of fish growth and production and 2) there is sub-regional variation in the response of fish and fish guilds to temperature, which is likely shaped by the local environmental and ecological dynamics. 

Barneche et al. have shown that fish reproductive output scales hypergeometrically with female weight. This data analysis is timely, relevant, and solid. Their results challenge the common fisheries management assumption that reproductive output is proportional to weight. However, we show that the implications of the result are much less dramatic than they are made to be. First, the graphic presentation created a distorted picture between the hypergeometric and the classic isometric description (see figure below). Second, their example for cod shows that current practice makes an error of 149%. By properly accounting for fish demography we show that the error is maximally on the order of 10%, and in most other fish stocks likely much less.

Ken H. Andersen, Nis Sand Jacobsen, P. Daniël van Denderen (2019) Limited impact of big fish mothers for population replenishment. Canadian Journal of Fisheries and Aquatic Sciences (link to the paper)

Why do we find primarily large pelagic predators such as tunas and billfish in the tropics, while in boreal and temperate regions large demersal species of gadoids and flatfish dominate? In a new paper we present the underlying factors determining the global distribution and productivity of these two groups of predatory fish.

The paper in Nature Ecology & Evolution is here: http://go.nature.com/2zHlOen
Original blog post can be found here.

Alone in a small boat fishing for marlin, Santiago in Hemmingway's The Old Man and The Sea thinks to himself in despair “My big fish must be somewhere”. The seemingly naïve but scientifically difficult question is naturally – where? We followed up on this question by investigating global patterns in big fish, focusing on why in the tropics we find primarily large pelagic predators such as tunas and billfish, while in boreal and temperate regions demersal species of gadoids and flatfish dominate.

To understand the variation in predatory fish across systems, we had to start looking at processes occurring at the base of marine food webs. Most energy in the oceans is generated by phytoplankton through photosynthesis. Phytoplankton is either consumed directly by zooplankton in the water column (pelagic pathway) or sinks to the bottom, where bottom-dwelling invertebrate organisms consume the dead and dying phytoplankton (benthic pathway). The downward flux of energy to the seafloor is influenced by environmental conditions, such as seabed depth, temperature and seasonality, and varies largely across systems.

We hypothesized that the relative strength of the pelagic and benthic energy pathway directly affects fish community structure and the production of large pelagic and demersal fish. This is well-known for some systems; for example, the shallow North Sea and Georges Bank have a strong benthic pathway and high diversity and production of demersal fish, whereas upwelling regions are characterized by a strong pelagic pathway and high pelagic fish production. Yet up to now, the importance of the pelagic and benthic energy pathways for the occurrence of large predatory fishes was unknown across systems on a global scale.

We used fisheries landings data to test the relative productivity of large marine teleost fishes across regions. Global fisheries landings are a unique source of data for fish on large spatial and temporal scales, but have to be used with care as it is difficult to grasp what the data actually tells you (the amount of fish in the sea, fisheries preference for a particular group of species, etcetera). After extensive checks of the data, we found that the fraction of large pelagic fish (relative to large demersal fish) in the catch is a meaningful proxy for what is in the sea. In a sense, we were lucky that the observed latitudinal patterns in pelagic and demersal fish predators are clear enough to withstand all robustness checks easily.   

So going back to Santiago’s search for big pelagic fish, we were able to show that the degree to which production is driven by energy from the pelagic ocean or from the benthic (bottom) community largely determines the type of predatory teleost fish in a system. This means that changes in the global occurrence and productivity of these large predatory fishes can be anticipated by understanding how climate change will affect the base of pelagic and benthic food chains.