February 2019: A recent paper in science have shown that fish reproductive output scales faster than proportional with female weight. This data analysis is timely, relevant, and solid. The results challenge the common fisheries management assumption that reproductive output is proportional to weight. However, in a recent comment in the Canadian Journal of Fisheries and Aquatic Sciences 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. Fish reproductive output may increase faster than proportionally with size. However, it does not matter much for the population-level reproductive output.
November 2017: "We have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates". Such wrote Darwin one and a half century ago. He was right -- see our new paper on climate change in a food-web context in Proc. Roy Soc. L. B (post print here) We show that species interactions — competition and predation — are more important processes than the increasing temperatures: apparently warm-adapted species are equally likely to be affected by climate change than cold-adapted species. Therefore, unfortunately, the effects of climate change cannot be predicted by the direct effects of temperature alone. In most cases we must account for changes in competitors and predators, which are much harder to predict.
Figure: The development of food-webs under climate change. Each dot represents a species and the colour shows its preferred temperature. The solid circles are species that are established in the food-web, and the lines represents their strength of interaction. As the temperature in the environment increases, some species may go extinct, or new species may invade (only in the left panel). Consequently, the structure of the food- web changes.
September 2017: Doctor defence "Fish and fisheries - the size- and trait-based approach" is now online: https://www.youtube.com/watch?v=OJXArL3TNg4
April 2017: Try the new version of the size spectrum fish community calculator here. The new version is written in R and includes 5 calibrated ecosystems.
November 2016: “What checks the natural tendency of each species to increase in numbers is most obscure”. Such wrote Charles Darwin one and a half century ago in “Origin of Species”. The statement is still true: empirical knowledge about density dependent regulation of animal populations is exceedingly difficult to obtain and interpret. In the new paper “When in life does density dependence occur in fish populations?” published in Fish and Fisheries, we develop the missing theoretic understanding about density dependent regulation in fish populations. Why care? We show that a correct understanding of density dependence is crucial for a central instrument in Maximum Sustainable Yield fisheries management – the minimum landing size regulations – that are used to avoid landing young fish by fisheries. See the paper in Fish and Fisheries here or download a preprint.
Fisheries yield vs. minimum landing size limit for four types of density-dependent regulation (thick lines) compared to the classic stock-recruitment theory that are used today (thin black dashed line). Fisheries yield is maximized with a size limit around size at maturation (vertical dotted line) only when density-dependent regulation occurs early.
July 2016: Sharks vs telests: How does the life-history strategy differ between elasmobranchs (sharks, rays etc) and teleosts (bony fish)? With respect to the size of their offspring, one difference is striking: elasmobranchs make offspring that is roughly a factor 100 (in weight) smaller than the adults, while most teleosts make offspring that is around 1 mg, independent of the adult size (see figure). We show that the difference in the offspring size strategy is unrelated to whether offspring is live or eggs, and there are no significant differences between growth, mortality etc. of elasmobranchs and teleosts. How, then, can evolution maintain these two very different offspring size strategies among very similar groups of organisms? In a recent paper in Journal of Theoretical Biology we develop an evolutionary model to show how the two offspring size strategies can co-exists evolutionary — in technical terms, we can say that there exists two local evolutionary stable states. The difference between the two strategies boils down to a difference in density-dependent control in the populations: if newly hatched offspring compete with one another, the evolutionary stable strategy is one where offspring size proportional to the adult size (the elasmobranch strategy), while if early life is devoid of density-dependent competition, the strategy with small offspring independent of adult size emerges.
Offspring size as a function of adult size. Open symbols are teleosts; closed are elasmobranchs.
2015: Characteristic sizes of life in the ocean: 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.
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.
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.