July 2015: Review: Characteristic Sizes of Life in the Oceans, from Bacteria to Whales (download here). Imagine picking a random organism out of the ocean. Which one trait characterizes the largest part of this organism’s physiology, encounter rate with prey (and predators), abundance of conspecifics, reproductive rate etc.: it’s body size. Body size also determines a large part of the organism’s fundamental design and strategy towards life: If it is small (unicellular) it is likely to photosyntheize, while very small bacteria do not photosynthesize. If the organism is large (larger than about a cm) it is likely to be slender and streamlined, while smaller organisms come in odd shapes with appendages necessary for sensing movements of the water to compensate for lack of vision. Larger organisms, such as fish, however, are largely visual predators. The largest organisms in the ocean are warm blooded and breathe air, while all organisms less than about 10 kg are cold blooded and take oxygen from the water.
The five aspects of pelagic marine life: body temperature, resource encounter strategy, mobility regime, sensing mode, and life history strategy. The art at the top represents the seven realms of life defined by body size. Artwork by Jan Heuschele.
We show how these and other differences in design and strategies of life in the ocean are due to physical and ecological processes dependent on body size, and we develop simple expressions to describe the characteristic body sizes where there is a transition from one type of life to another, i.e., between bacteria, phytoplankton, mixotrophs, unicellular and multicellular zooplankton, fish, and marine mammals. In this way we stengthen the fundamental theoretical framework about body size, initiated by some of the founding fathers of evolutionary and ecological thinking, Haldane and Elton.
December 2014: New paper: Trade-offs between objectives for ecosystem management of fisheries. Gedankenexperimente in ecosystem-based fisheries management. What happens to the ecosystem if we optimise the fishing pattern for yield of biomass (MSY) or for economic rent (EMSY)? And how does that tie in with conservation constraints? Using a trait-based size spectrum model we have been able to extend this classical analysis from a single species to an entire fish community.
The size distribution (top) and the fishing pattern of three fishing fleets (bottom) when fishing to maximize economic rent.
May 2014: New paper: A life-history evaluation of the impact of maternal effects on recruitment and fisheries reference points. The Big Old Fecund Females (the "BOFFs") in a fish stock have been shown to produce more viable eggs and larvae than younger females. Should we take this into account when we evaluate the producitivity and resilience of a fish stock? Using a trait-based model of fish stock we show that even though the BOFFs produce more viable offspring their impact on the recruitment of the entire stock is small. The reason is that the abundance of younger fish is so much larger than the abundance of BOFFs that the main contribution to recruitment comes from the young fish. The only exception is fish stocks which are particularly vulnerable to fishing. For those stocks the recruitment from BOFFs is an important contribution to the resilience of stock.
December 2013: New paper The consequences of balanced harvesting of fish communities. How shall we manage fisheries within an ecosystem approach to fisheries? A recently proposed solution is to make a "balanced" harvesting of ecosystem components. Balanced harvesting implies that small individuals should be fished harder than large individuals. We make an assessment of balanced harvesting using the size- spectrum modelling concept. We find that balanced harvesting can produce as much biomass yield as current fishing patterns, and it may even do so at a lower risk of compromising other ecosystem components. The price to pay is that the catch is comprised mainly of small individuals. Balanced harvesting can therefore be viewed an attempt to maximise the yield from a forage fishery at the expense of yield from the consumer fishery.
October 2013: Updated the single-species size-spectrum simulator. Now with possibility to simulate "balanced fishing".
The fishing mortality giving the maximum yield (Fmsy) as a function of the maximum size of fish in a population (Woo) (black line). The black dots are Fmsy for selected fish stocks from official ICES assessments. The grey areas represent runs with random parameters of the model (see paper for details).
December 2012: New paper: Control of plankton seasonal succession by adaptive grazing.
March 2012: New paper: Trait diversity promotes stability of community dynamics. In this paper we show that the dynamic solutions of the community size spectrum model are unrealistic. We do this by comparing the community model with the same model where we have added trait diversity. The model with trait diversity has dynamic solutions with much smaller amplitude than the community model (see below). These results are important because they demonstrate the limitations of the community size-spectrum model. My student, Lai Zhang, performed the technically demanding calculations behind this exercise.
Phase plot of predator biomass as a function of prey biomass for the simple community size-spectrum model (grey) and the same model where the diversity of individuals is accounted for (black). The basic model has chaotic dynamics and very large amplitude variation whereas the model with trait diversity has regular and low amplitude oscillations.