Evaluating effort regulation in mixed fisheries: a Monte Carlo approach

This paper evaluates whether effort regulation could achieve the goal of protecting low-abundance species in mixed fisheries. We construct a two-species bio-economic model and compare the stock abundance ratio in the end of the fishing season with the ratio prior to the fishing. Fishers’ profit maximization problem is governed by three key factors: (a) the overall efficiency of catching different species (catchability), (b) the price of different species, and (c) their ability to catch the favoured species separately from the less-favoured species (separability). Using a Monte Carlo sampling of feasible parameters space, we show that effort regulation has good chances (87% of the cases) of maintaining the end stock ratio near equal levels ( 1/2 < stock ratio < 2) when the initial stock ratio is equal. If the initial stock ratio is not equal, however, there is a high risk (about 50% of the cases) that effort control increases differences in the relative species abundances, rather than diminishing them. The effects depend on whether the key factors determining fishing profitability are counteracting or reinforcing each other, and their relative strength. Our results warn against placing too much faith on the ability of effort regulation to protect species at low abundances from excessive exploitation.submittedVersio

Ecological and evolutionary effects of harvesting: lessons from the candy-fish experiment

Understanding the challenges of sustainable fisheries management is not easy for non-specialists, and even many specialists fail to appreciate the potential evolutionary consequences of harvest. We propose candy-fish experiments as a savoury approach to teaching and disseminating the key principles of applied ecology and evolution to students, practitioners and the general public. We performed a simple experiment where the resource was represented by fish-shaped candy of distinct colours and flavours (strawberry and liquorice). Typically, harvesting was neither ecologically sustainable (55% of the populations were extinct by the end of the experiment) nor evolutionarily sustainable (most surviving populations had liquorice fish only). This harvest-induced evolution went apparently unnoticed. Somewhat encouragingly, the harvest was most likely ecologically sustainable when a person spontaneously took the role of a stock manager.acceptedVersio

The importance of social dimension and maturation stage for the probabilistic maturation reaction norm in Poecilia reticulata

Maturation is an important event in an organism's life history, with important implications on dynamics of both wild and captive populations. The probabilistic maturation reaction norm (PMRN) has emerged as an important method to describe variation in maturation in wild fish. Because most PMRNs are based on age and size only, it is important to understand limitations of these variables in explaining maturation. We experimentally assessed (i) the sensitivity of age- and size-based PMRNs to unaccounted sources of plasticity, (ii) the role of social environment on maturation and (iii) the significance of estimating PMRNs early and late in the maturation process (initiation and completion of maturation, respectively). We reared male guppies (Poecilia reticulata) under laboratory conditions, subjected to two food levels and three different social cues. We found that growth and social environment affected the maturation in a way that could not be accounted for by their effect on age and size. PMRNs estimated for the initiation stage were less plastic (growth differences and social cues influenced the PMRN shape only little) than those for completion. The initiation of maturation is probably closer to the maturation ‘decision’ and allows determining factors influencing maturation decision most accurately.acceptedVersio

The importance of social dimension and maturation stage for the probabilistic maturation reaction norm in Poecilia reticulata

Maturation is an important event in an organism's life history, with important implications on dynamics of both wild and captive populations. The probabilistic maturation reaction norm (PMRN) has emerged as an important method to describe variation in maturation in wild fish. Because most PMRNs are based on age and size only, it is important to understand limitations of these variables in explaining maturation. We experimentally assessed (i) the sensitivity of age- and size-based PMRNs to unaccounted sources of plasticity, (ii) the role of social environment on maturation and (iii) the significance of estimating PMRNs early and late in the maturation process (initiation and completion of maturation, respectively). We reared male guppies (Poecilia reticulata) under laboratory conditions, subjected to two food levels and three different social cues. We found that growth and social environment affected the maturation in a way that could not be accounted for by their effect on age and size. PMRNs estimated for the initiation stage were less plastic (growth differences and social cues influenced the PMRN shape only little) than those for completion. The initiation of maturation is probably closer to the maturation ‘decision’ and allows determining factors influencing maturation decision most accurately.acceptedVersio

What can selection experiments teach us about fisheries-induced evolution?

There is evidence that fisheries are altering the phenotypic composition of fish populations, often in ways that may reduce the value of fish stocks for the exploiters. Despite the increasing number of theoretical and field studies, there is still debate as to whether these changes are genetic, can be reversed, and are occurring rapidly enough to be considered in fisheries management. We review the contribution that selection experiments have already provided with respect to the study of the evolutionary effect of fisheries, identify issues that still require more study, and outline future directions for doing so. Selection experiments have already been crucial in showing that harvesting can lead to phenotypic and genetic evolution over relatively short time frames. Furthermore, the experiments have shown the changes involve many other traits than those under direct selection, and that these changes tend to have population-level consequences, including a decreasing fisheries yield. However, experiments focused on fisheries-induced evolution that fulfil all our requirements are still lacking. Future studies should have more controlled and realistic set-ups and assess genetic changes in maturation and growth (i.e. traits most often reported to change) to be more relevant to exploited populations in the wild.acceptedVersio

The importance of social dimension and maturation stage for the probabilistic maturation reaction norm in Poecilia reticulata

Maturation is an important event in an organism's life history, with important implications on dynamics of both wild and captive populations. The probabilistic maturation reaction norm (PMRN) has emerged as an important method to describe variation in maturation in wild fish. Because most PMRNs are based on age and size only, it is important to understand limitations of these variables in explaining maturation. We experimentally assessed (i) the sensitivity of age- and size-based PMRNs to unaccounted sources of plasticity, (ii) the role of social environment on maturation and (iii) the significance of estimating PMRNs early and late in the maturation process (initiation and completion of maturation, respectively). We reared male guppies (Poecilia reticulata) under laboratory conditions, subjected to two food levels and three different social cues. We found that growth and social environment affected the maturation in a way that could not be accounted for by their effect on age and size. PMRNs estimated for the initiation stage were less plastic (growth differences and social cues influenced the PMRN shape only little) than those for completion. The initiation of maturation is probably closer to the maturation ‘decision’ and allows determining factors influencing maturation decision most accurately

Fisheries-induced evolution

Increased mortality from fishing is expected to favor faster life histories, realized through earlier maturation, increased reproductive investment, and reduced postmaturation growth. There is also direct and indirect selection on behavioral traits. Molecular genetic methods have so far contributed minimally to understanding such fisheries-induced evolution (FIE), but a large body of literature studying evolution using phenotypic methods has suggested that FIE in life-history traits, in particular maturation traits, is commonplace in exploited fish populations. Although no phenotypic study in the wild can individually provide conclusive evidence for FIE, the observed common pattern suggest a common explanation, strengthening the case for FIE. This interpretation is supported by theoretical and experimental studies. Evidence or FIE of behavioral traits is limited from the wild, but strong from experimental studies. We suggest that such evolution is also common, but has so far been overlooked

Fisheries-induced evolution

Increased mortality from fishing is expected to favor faster life histories, realized through earlier maturation, increased reproductive investment, and reduced postmaturation growth. There is also direct and indirect selection on behavioral traits. Molecular genetic methods have so far contributed minimally to understanding such fisheries-induced evolution (FIE), but a large body of literature studying evolution using phenotypic methods has suggested that FIE in life-history traits, in particular maturation traits, is commonplace in exploited fish populations. Although no phenotypic study in the wild can individually provide conclusive evidence for FIE, the observed common pattern suggests a common explanation, strengthening the case for FIE. This interpretation is supported by theoretical and experimental studies. Evidence for FIE of behavioral traits is limited from the wild, but strong from experimental studies. We suggest that such evolution is also common, but has so far been overlooked.submittedVersio

The effects of selective harvest on Japanese Spanish mackerel (Scomberomorus niphonius) phenotypic evolution

Japanese Spanish mackerel (Scomberomorus niphonius) is an important fish species in the China Seas with wide distribution, extensive migration, and high economic value. This species has been yielding high fisheries production despite experiencing continuously high fishing pressure and the conversion from gillnet to trawl harvesting. Meanwhile, changes in life-history traits have been observed, including earlier maturation and smaller size at age. Here, we build an individual-based eco-genetic model parameterized for Japanese Spanish mackerel to investigate the population’s response to different fishing scenarios (fishing by trawl or by gillnet). The model allows evolution of life-history processes including maturation, reproduction and growth. It also incorporates environmental variability, phenotypic plasticity, and density-dependent feedbacks. Our results show that different gear types can result in different responses of life-history traits and altered population dynamics. The population harvested by gillnet shows weaker response to fishing than that by trawl. When fishing ceases, gillnet-harvested population can recover to the pre-harvest level more easily than that harvested by trawl. The different responses of population growth rate and evolution to different fishing gears demonstrated in this study shed light on the sustainable management and utilization of Japanese Spanish mackerel in the over-exploited China Seas.publishedVersio

Gaussian mixture models reveal highly diverse targeting tactics in a coastal fishing fleet

Fishermen make repeated choices with respect to when, where, and how to catch their target species. While these targeting tactics—and the factors shaping them—are known to fishers and some experts, knowledge about them is largely informal and not well utilized for management purposes. To formalize information on targeting tactics, we propose a set of methods combining model-based classification of target species with generalized linear models. We apply these methods to Norwegian coastal fishing vessels that caught Atlantic cod (Gadus morhua) as a part of their catch portfolio in 2019. The data contains nearly 32000 fishing trips by 761 vessels. Gaussian mixture models identify eight latent targeting tactics. Cod contributes significantly to three of the tactics. The Herfindahl–Hirschman Index, a measure of vessel-level diversity of tactics, shows that one quarter of the vessels had a specialized strategy (targeting cod plus at most one additional tactic). While cod is often studied as a single-species fishery, we show that cod-catching vessels can be engaged in relatively pure fisheries during some fishing trips but switch to different, often more mixed targets during other trips. We term this as “sequential mixed fisheries”. This is both a challenge and an opportunity for the fisheries management.publishedVersio