Fishes in a changing world: learning from the past to promote sustainability of fish populations.

Populations of fishes provide valuable services for billions of people, but face diverse and interacting threats that jeopardize their sustainability. Human population growth and intensifying resource use for food, water, energy and goods are compromising fish populations through a variety of mechanisms, including overfishing, habitat degradation and declines in water quality. The important challenges raised by these issues have been recognized and have led to considerable advances over past decades in managing and mitigating threats to fishes worldwide. In this review, we identify the major threats faced by fish populations alongside recent advances that are helping to address these issues. There are very significant efforts worldwide directed towards ensuring a sustainable future for the world's fishes and fisheries and those who rely on them. Although considerable challenges remain, by drawing attention to successful mitigation of threats to fish and fisheries we hope to provide the encouragement and direction that will allow these challenges to be overcome in the future.

Why the damselfish Chromis chromis is a key species in the Mediterranean rocky littoral - a quantitative perspective.

The damselfish Chromis chromis is typically the most abundant fish species in the rocky littoral environment of the Mediterranean Sea, where it feeds in huge shoals on incoming zooplankton and rests near the seabed each night. Research has shown that large populations of C. chromis play a fundamental role by transferring carbon, nitrogen and phosphorus directly from the pelagic system to the littoral in the form of solid and liquid wastes and therefore that this species acts as a vital conduit for nutrients. In addition, C. chromis represents a major prey item for predatory fish and seabirds, it has been shown to exert significant predation pressure on coastal zooplankton populations, but it is also a major consumer of fish eggs and hence may have a strong influence on the dynamics of other fish species. In this review paper the contention is that C. chromis represents a key species in the Mediterranean Sea rocky littoral. This view is supported by quantitative evidence that has been amassed over a number of years, but is collated here for the first time. Chromis chromis is of only minor importance to commercial fisheries (it is usually discarded), but trends in population abundance of this species can be indicative of levels of fishing pressure on other commercial species. It has been demonstrated that where predator numbers are able to recover from intensive fishing (e.g.in marine protected areas; MPAs), this is often mirrored by lower densities of C. chromis. Several papers have championed C. chromis as a possible indicator species for human disturbance. As well as being directly and indirectly affected by fishing, the species is also sensitive to a number of other anthropogenic pressures, most notably poor water quality and noise pollution.

Fisheries, low oxygen and climate change: how much do we really know?

As a result of long-term climate change, regions of the ocean with low oxygen concentrations are predicted to occur more frequently and persist for longer periods of time in the future. When low levels of oxygen are present, this places additional pressure on marine organisms to meet their metabolic requirements, with implications for growth, feeding and reproduction. Extensive research has been carried out on the effects of acute hypoxia, but far less on long-term chronic effects of low oxygen zones, especially with regard to commercially important fishes and shellfishes. To provide further understanding on how commercial species could be affected, the results of relevant experiments must support population and ecosystem models. This is not easy because individual effects are wide-ranging; for example, studies to date have shown that low oxygen zones can affect predator-prey relationships as some species are able to tolerate low oxygen more than others. Some fishes may move away from areas until oxygen levels return to acceptable levels, while others take advantage of a reduced start response in prey fishes and remain in the area to feed. Sessile or less mobile species such as shellfishes are unable to move out of depleted oxygen zones. Some species can tolerate low oxygen levels for only short periods of time, while others are able to acclimatize. To advance the knowledge-base further, a number of promising technological and modelling-based developments and the role of physiological data within these, are proposed. These include advances in remote telemetry (tagging) and sensor technologies, trait-based analyses to provide insight into how whole assemblages might respond in the future, research into long-term adaptability of species, population and ecosystem modelling techniques and quantification of economic effects. In addition, more detailed oxygen monitoring and projections are required to better understand the likely temporal and local-scale changes in oxygen.

A trans-Atlantic examination of haddock Melanogrammus aeglefinus food habits.

The food habits of Melanogrammus aeglefinus were explored and contrasted across multiple north-eastern and north-western Atlantic Ocean ecosystems, using databases that span multiple decades. The results show that among all ecosystems, echinoderms are a consistent part of M. aeglefinus diet, but patterns emerge regarding where and when M. aeglefinus primarily eat fishes v. echinoderms. Melanogrammus aeglefinus does not regularly exhibit the increase in piscivory with ontogeny that other gadoids often show, and in several ecosystems there is a lower occurrence of piscivory. There is an apparent inverse relationship between the consumption of fishes and echinoderms in M. aeglefinus over time, where certain years show high levels of one prey item and low levels of the other. This apparent binary choice can be viewed as part of a gradient of prey options, contingent upon a suite of factors external to M. aeglefinus dynamics. The energetic consequences of this prey choice are discussed, noting that in some instances it may not be a choice at all.

Use of morphological characteristics to define functional groups of predatory fishes in the Celtic Sea.

An ecomorphological method was developed, with a focus on predation functions, to define functional groups in the Celtic Sea fish community. Eleven functional traits, measured for 930 individuals from 33 species, led to 11 functional groups. Membership of functional groups was linked to body size and taxonomy. For seven species, there were ontogenetic changes in group membership. When diet composition, expressed as the proportions of different prey types recorded in stomachs, was compared among functional groups, morphology-based predictions accounted for 28-56% of the interindividual variance in prey type. This was larger than the 12-24% of variance that could be explained solely on the basis of body size.

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes.

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ13C and δ15N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ13C of plants ranged from -8.59‰ to -33.74‰, of benthic invertebrates from -17.0‰ to -20.52‰, of planktonic invertebrates from -20.08‰ to -22.34‰ and of fishes from -16.27‰ to -19.59‰. δ15N was generally greater at higher trophic levels. δ15N of plants was 0.95-2.92‰, of benthic invertebrates 1.69-6.54‰, of planktonic invertebrates 3.51-6.82‰ and of fishes 4.63-9.77‰. 13C enrichment tended to be associated with benthic food chains and 13C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ15N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected.