Ecosystem Effects of Fishing

16 Ecosystem Effects of Fishing

M.J. KAISER AND S. JENNINGS

16.1 INTRODUCTION

species may be predators, grazers, prey, scavengers and competitors. Some species may form habitat

Humans are extremely effective predators. As a structures (Lenihan 1999) or maintain habitat result we have seen at various times throughout patchiness through their feeding activities (Van history the sequential overexploitation of many Blaricom 1982). The consequences of harvesting a target species (FAO 1994). Traditional fisheries particular species will depend on its role and domi- management focused on these single species with nance within an ecosystem and also the com-

a recent move towards multispecies approaches. plexity of the species interactions in that system It has proved difficult enough to manage stocks of (Pauly and Christensen, Chapter 10, this volume). single species successfully, and so it is not difficult Hence intensive harvesting of one species might to understand why relatively little effort has been relieve predation on its prey species while harvest- devoted to trying to predict ecosystem responses ing prey species might increase predation pressure to target-species removal. Nevertheless, the wider on alternative prey types. When they exist, such ecosystem effects of fishing activities are becom- ecosystem responses to single-species removal ing more apparent, which emphasizes our need to are known as ‘trophic cascades’ (Pace et al. 1999). understand the consequences of exploiting marine Later in this chapter we give some examples where resources (Hall 1998; Jennings and Kaiser 1998; trophic cascades have resulted from fishing activi- Auster and Langton 1999). Although we reviewed ties. However, a recent study by Yodzis (1998) this topic in detail several years ago (Jennings and demonstrated that even when dealing with a well- Kaiser 1998), there have been new and exciting de- quantified ecosytem such as the Benguela system, velopments in the interim period, some of which the predicted outcome of culling large top pre- have confirmed our previously unsubstantiated dators such as seals can be highly variable (see theories.

Section 16.5).

While intensive fishing has led to well- documented population reductions of many target species (e.g. Davis et al. 1996; Myers et al. 1996), it

16.2 EFFECTS OF

has also caused changes in the species composi-

HARVESTING

tion structure within fish communities (e.g.

TARGET SPECIES

Greenstreet and Hall 1996). Whereas in the past fish assemblages had a broad range of size classes,

A target species is just one component of an exploited assemblages tend to be dominated by a ecosystem; hence it may perform a variety of much higher proportion of small-sized individuals functions as it interacts with other species. Target (Kirkwood et al. 1994; Pauly et al. 1998). Jennings

343 et al. (1998) used a comparative approach based on (Carangidae) declined in abundance rapidly as fish-

Ecosystem Effects of Fishing

phylogenetic comparisons to examine the differ- ing pressure increased and recovered slowly when ential effects of fishing on individual species that fishing ceased. During periods when the commu- have contrasting life histories. They found that nity was most intensively exploited, they formed a those fishes that decreased in abundance relative smaller proportion of the total abundance of the to their nearest phylogenetic relative matured community and the community was dominated later and at a greater size, grew more slowly to- by smaller species with faster life histories (see wards a greater maximum size and had lower also Polunin, Chapter 14, this volume). rates of potential population increase (Table 16.1).

Thus the response of fish populations to ex- For example, common skate (Raja batis) have ploitation follows a consistent pattern, with the the steepest slope for the decline in catch rates progressive removal of the largest body-sized fauna through time, whereas the smaller starry rays from the system leading to dominance by smaller- (Raja radiata) actually show an increase in popula- bodied individuals. Consequently, the sustained tion numbers. While both halibut (Hippoglossus harvesting of fish populations has led to a gradual hippoglossus ) and long rough dabs Hippoglos- decline in their body-size spectrum (Rice and soides platessoides have decreased with time, Gislason 1996). This has important considerations the rate of decrease is three times greater for the for predator–prey interactions. For example, juve- halibut that attains a much greater maximum nile cod, Gadus morhua, are preyed upon by adult size.

whiting, Merlangius merlangus, while the latter

Patterns observed in these North Sea studies are consumed by adult cod. A decrease in the body- are also typical of those in exploited reef fish size spectrum of cod populations could mean that communities. Russ and Alcala (1998a,b) looked at they are exposed to proportionately greater levels changes in a Philippine reef fish community dur- of predation. Conservation measures designed ing periods of exploitation and protection from to reduce bycatches of whiting in the cod fishery fishing. Large predatory species with slow life could further exacerbate this problem by further histories, such as snappers (Lutjanidae), emperors reducing the body-size spectrum of cod through (Lethrinidae), sea basses (Serranidae) and jacks harvesting while conserving the population struc-

Table 16.1 Abundance trends and life-history parameters for selected species in the North Sea fish community. Species

K (y -1 ) T m (y) L m (cm) Raja batis

Common name

Trend

L • (cm)

0.06 11.0 130 Raja naevus

Common skate

92 0.11 9.0 59 Raja radiata

Cuckoo ray

66 0.23 4.0 46 Squalus acanthias

Starry ray

90 0.15 6.5 67 Scyliorhinus canicula

Spurdog

88 0.20 5.0 58 Trisopterus minutus

Lesser-spotted dogfish

20 0.51 2.0 15 Trisopterus esmarkii

Poor cod

23 0.52 2.3 19 Hippoglossus hippoglossus

Norway pout

0.10 5.8 83 Hippoglossoides platessoides

25 0.34 2.6 15 Notes : Trend: slope of linear relationship between standardized catch rate (numbers h -1 ) in standard fisheries sur-

Long rough dab

veys and time (years); L • : asymptotic (maximum) length; K: growth rate; T m : age at maturity; L m : length at maturity. The sequence and grouping of species reflects their phylogenetic relationship. The species are shown in phylogeneti- cally linked couplets or triplets and the first species is the one for which catch rate has declined the most. (Source: adapted from Jennings et al. 1999.)

Chapter 16

ture of whiting through technical conservation Urchins erode the reef matrix as they graze and measures.

this prevents the settlement and growth of coral re- In the next few sections we give some examples cruits. Occasionally recruitment failure or urchin of where cascade effects have and have not been disease may lead to a collapse of urchin popula- observed in marine systems in response to the tions. In the absence of such occurrences, inter- selective removal of target species.

vention may be required to promote recovery of the reef ecosystem by the deliberate removal of

16.2.1 Exploitation of target

urchins. McClanahan et al. (1996) found that when

species on tropical reefs they removed urchins from unfished experimental

plots on Kenyan reefs, there were significant in- Most tropical reefs are fished intensively since creases in algal cover and fish abundance within they provide the main sources of protein and in- one year. However, on fished reefs, herbivorous come for fishers who have few other means of gen- fishes were less abundant, and the algae rapidly erating a living. Many trophic groups are targeted, overgrew corals as they proliferated in the absence and the abundance of herbivorous, piscivorous or of herbivores. invertebrate feeding fishes is often reduced by an

While many studies have shown that the abun- order of magnitude or more on fished reefs (Russ dance of piscivorous reef fishes is greatly reduced 1991).

by fishing, there is little evidence for a correspond- Fishes and sea urchins are the most abundant ing increase in the abundance of the fish species herbivores on most tropical reefs. Sea urchin abun- that are their prey (Bohnsack 1982; Russ 1985; dance is regulated by recruitment success, food Jennings and Polunin 1997). Why is the response supply and natural mortality due to predation by of prey fish communities so weak? The reasons for fish and disease. The main predators of sea urchins this are probably linked to reef fish community are invertebrate feeding fishes such as emperors structure, in which phylogenetic groupings of fish (Lethrinidae) and triggerfishes (Balistidae) and, contain many species, with a wide range of life on reefs in the Caribbean and East Africa, fish history traits, behavioural differences and feeding predation strongly regulates urchin populations strategies (Hiatt and Strasburg 1960; Parrish et al. (McClanahan 1995a). As a result, on reefs where 1985, 1986; Hixon 1991). Moreover, most fish populations of emperors and triggerfishes have species alter their feeding behaviour and hence been reduced by fishing, there is good evidence diet as they grow and can act as both prey and that urchins have proliferated (McClanahan and predators of other species. Hence, while the collec- Muthiga 1988; McClanahan 1992, 1995b).

tive impacts of predatory fish are large, the impacts When urchins dominate herbivore biomass, of individual predator species on the dynamics of they will graze the majority of algal production on their prey are minor (Hixon 1991).

a reef. Because urchins have low consumption and Notably, at smaller scales (m 2 compared with respiration rates they can survive and reproduce km 2 ) there is some evidence for the role of pre- when their food supply is greatly reduced by their dation as a structuring force, particularly when own feeding activities. In contrast, herbivorous habitat or refuge space is directly limited. Thus fishes have higher consumption and respiration experimental reductions in piscivore abundance rates and are unable to tolerate low levels of food led to detectable decreases in the abundance and availability. As a result, urchins outcompete her- diversity of their prey (Caley 1993; Hixon and bivorous fishes and reach maximum biomass Beets 1993; Carr and Hixon 1995). levels an order of magnitude higher (McClanahan 1992). Since the herbivorous fishes are poor com-

16.2.2 Removal of predatory fish in

petitors, they may be unable to achieve their former abundance when fishing is stopped

temperate marine fisheries

(McClanahan 1995a). Even in the most intensively fished marine ecosys- (McClanahan 1995a). Even in the most intensively fished marine ecosys-

Elsewhere, we have concluded that there are relatively few circumstances in which changes in the abundance of piscivorous fishes in marine ecosystems has had cascading impacts on other parts of the system (Jennings and Kaiser 1998). Most fish species can act as both predators and prey in the course of their life history and adult pre- dators are capable of switching diet and their feeding behaviour in response to prey availability (Mittlebach, Chapter 11, Volume 1). Hence high fish consumption rates do not necessarily imply that predation is a structuring force of fish popula- tions within a particular system. Indeed, in tem- perate systems, the strongest evidence for the predator-based control of prey species comes from the impact of humans on their target species. The strength of this relationship is likely to result from the conservative fishing strategies employed by humans. In the majority of commercial fisheries, fishers are unwilling to be flexible in their aims and target a relatively small proportion of the total fish fauna. Most predatory fish, conversely, are very gen- eralist feeders, often switching to invertebrate prey or cannibalism and eating many species of fishes at different stages in their life history (Fig. 16.1).