6.
7 Fisheries and aquaculture
6.7.1 Wild fisheries
Worldwide, the main estuarine fisheries are for shellfish, with the collection of some of
the abundant natural populations of invertebrates, such as shrimps, crabs, oysters, cockles and
mussels. Increasingly the catching of natural stocks is supplemented by mariculture, most
notably for oysters and mussels. Bait digging by sport fishermen occurs widely in estuaries, and
may involve considerable disruption of the intertidal fauna.
In addition to the shellfish industries are the vertebrate fisheries. While many of the fish,
which enter estuaries are not commercially exploited because they are the nursery stocks, or
unwanted species, other species are heavily exploited. Salmon, sea trout, and eels all pass
through estuaries on route from the sea to rivers, and many commercial fisheries exploit them.
The Gulf of Mexico, stretching from Florida to Texas and Mexico, is a key fishery area for the
United States, with 28% of their total fish landings coming from the brackish bays and lagoons
of this area. The catch is mainly menhaden (Brevoortia spp.), striped mullet (Mugil cephalus),
croaker (Micropogon undulatus) plus Penaeid shrimps, blue crab, and oysters. All except the
croaker are estuarine species, which in general spawn at sea. The larvae enter estuaries, grow
there, and when fully mature return to the sea. Other rich estuarine fisheries are at the mouth of
the Amazon, in Nigerian estuaries, and in Indianestuaries, such as the Ganges, where
estuarineconditions extend 160 km upstream from the mouth.
Cumulative fish species recorded in tidal Thames (FulhamTilbury)
Figure 6.9 Number of fish species recorded in the Thames estuary, England. Drawn from data
supplied by Dr. S. Colclough, Environment Agency, London.
As with all types of fishing, there are now major concerns on the ecosystem effects of the
activities, these have been summarized in Fig. 6.10. For example, as well as the removal of target
and nontarget fish and shellfish, fishing activity in estuaries can disturb or destroy benthos and
thus affect their dependent fish and bird populations (Jennings and Kaiser 1998; Hall 1999;
Blaber et al. 2000). The main effects of fishing are:
(1) to cause local extinctions, where there is a large-scale removal of a particular species, often
as the target species, for example, the smelt, Osmerus eperlanus, which disappeared from the
Forth Estuary, Scotland in the middle of the twentieth century mostly as the result of overfishing;
(2) to affect the population viability, genetics, and maturity of the target organisms, for example,
the removal of spawning stocks through over-fishing of pelagic and demersal populations;
(3) to affect nontarget organisms, both other fishes, such as the taking of juvenile herring during
estuarine fishing for sprat, and other species such as porpoises taken in estuarinen salmon
netting; (4) to affect the nursery function, such as the removal of large numbers of juvenile plaice
and dab during shrimp fishing, and by removing the mudflat habitat used by juveniles;
(5) to affect trophic interactions such as the removal of prey fishes, such as sandeels taken by
seabirds and crustaceans, or the shrimps, which are central to the functioning of many estuarine
food webs, and by changing food webs by increasing scavengers, especially benthic megafauna,
following seabed damage by fishing gear and the production of detrital bycatch;
(6) by habitat modification and destruction, through land-claim and infrastructure creation (the
building of ports and harbours) and by changes to substratum integrity by trawling;
(7) by water quality effects, which may occur through bed resuspension caused by trawling, the
input of other introduced materials such as litter, and through organic enrichment produced by
discards.
Many of the problems caused by marine fisheries occur in estuarine and wetland habitats
although there are few case studies. Elliott et al. (1990) suggests an example of a marine fishery
affecting estuarine fishes where the coastal and marine cod fisheries appeared to affect the
nursery function of estuaries. In the Forth estuary, eastern Scotland, the decline of juvenile cod
appeared to reflect the increasing North Sea catches of the larger, reproducing stocks. This
feature was also shown in the Tyne estuary, NE England.
Each type of fishing will have an ecosystem effect and so, as an example, trawl fishing in
temperate estuaries is considered here (Fig. 6.11). The summary (Conceptual Model)
concentrates on the effects of trawling and its related activities (port and harbors, litter
production and gear loss). This conceptual model summarizes the effects on the following
aspects:
noncommercial sizes (especially juveniles and thus the nursery function of estuaries)
noncommercial species (the remainder of the assemblage and the effects of discards and
individual damage)
target species (through community and population changes at the ecological and genetic levels,
and the effects on competing predators)
the physical integrity of the system (through bed disturbance and damage to the benthos)
contamination of the estuarine system (through discharges of soluble pollutants and large and
small particulate materials, including litter and gear loss)
the creation of infrastructure (habitat loss, especially of highly productive intertidal areas). At
present, there is no attempt to quantify the links within the conceptual model or to assess the
geographical extent of the effects and so the model remains qualitative rather than quantitative.
Figure 6.10 The effects of fisheries on estuarine ecosystems. (From Elliott and Hemingway 2002.)
6.7.2 Aquaculture (culture offinfish and shellfish)
Since the 1980s there has been the rapid growth of mariculture in many countries. In
Scotland, Norway, and Canada the main expansion has been in the rearing of salmon in floating
cages (Fig. 6.12). In assessing the effects of aquaculture, it is necessary to consider the areas
used, the conditions required by the fish farmers or encountered in those areas, the effects on the
water column, sediment and benthos, the impacts on local fishes and birds, the impacts at
different biological levels, and the addition of chemicals required for efficient production. Each
of these requires management strategies and control and regulation. Within Europe, the major
areas used are along the Scottish West Coast and island margins, the Irish West Coast and the
Norwegian fjords. The areas are in transitional water bodies such as estuaries, fjords, and sea
lochs and they require certain facilities such as access, good water quality, and low to moderate
water flow (low energy hydrophysical regime). Because of the latter, the areas are likely to be
stratified. In essence, the effects result from the nature of containing and feeding the fish, for
example, salmon are usually fed on pelleted food, composed mainly of fish meal, and up to 20%
of this food may not be intercepted and will fall to the bottom, along with the feces produced by
the fish (Fig. 6.13). Since the fish cages are usually placed in shallow waters, the waste material
rapidly falls to the bottom and accumulates there.
In temperate areas, cages retaining fishes and trays or ropes supporting bivalve shellfish
and in tropical areas, cages retaining crustaceans, mainly prawns, are kept within the water
column. This produces water column effects through the addition of wastes (as particulate
organic matter (POM), dissolved organic matter (DOM), excess food, feces, mucus, and
detritus). The wastes will add nutrients thus having the potential to change oligotrophic (nutrientlimited)
systems
to
more nutrient rich ones. Naturally nutrientrich systems (hypernutrified) can become eutrophic.
There may be a reduction of Dissolved Oxygen (by fin/shell-fish demand and the Biochemical
Oxygen Demand of the wastes) and the potential for red-tides (noxious microalgae blooms) has
been suggested for the Scottish West Coast. There will be a hydrographic disruption by the
structures (they will impede water flow, especially where there is excessive weed attachment)
and the inputs from the caged organisms will lead to microbial change through and input of fecal
microorganism input. Finally, there may be an increase in suspended solids hence an increased
turbidity, leading to potential for a reduction in primary productivity. Many of these effects fall
within the generally acknowledged symptoms of eutrophication.
Figure 6.11 The effects of trawl fishing on the estuarine ecosystem. (From Elliott and Hemingway 2002.)
Figure 6.12 A floating sea-cage fish-farm for salmonid fish (salmon or trout). Such farms have
become common sights on the fjordic estuaries of Norway, Ireland, Canada, New Zealand,
Scotland, and elsewhere. (See also Fig. 6.13.)
Figure 6.13 The environmental impact of the floating-cage culture of salmonid fish. Values are
expressed in terms relating to the production of 1 tonne of fish. (After Gowen et al. 1988.)
Arguably the greatest impact will be in changes to the sediment and benthos. There will
be an accumulation of solid wastes on the bed given that 510% of food is wastage and 2530%
of the food weight will be excreted as feces. In the case of bivalve shellfish there will be the
biodeposition of sediments through pseudofeces production, hence changing the nature of
underlying sediments. The hydrodynamic change because of impeded water flow can lead to
sediment structure change through the production of a low energy area. The organic input and
enrichment of sediment will produce anaerobic sediments, which may ultimately affect the water
quality through the production of hydrogen sulfide (H2S) and methane (CH4), which in turn are
toxic. Anoxic sediments will affect the water column if the Redox Potential Discontinuity, as the
boundary between the upper oxygenated levels and the lower anaerobic levels, migrates out of
the sediments. This is then reflected by the development of Beggiatoa mats utilizing the
hydrogen sulfide. In turn, the anaerobic conditions lead to the development of opportunist
populations and thus benthic prey quality palatability has been changed for fish and megafaunal
predators. Brown et al. (1987) have shown that underneath the floating salmon cages the
sediment is highly reducing (negative Eh) and azoic. A highly enriched zone, dominated by the
opportunistic and pollution tolerant polychaetes C. capitata and S. fuliginosa, occurred from the
edge of the cages out to approximately 8 m. A slightly enriched zone occurred at up to 25 m,
beyond which the fauna was unaffected by the cages. Thus the ecological effects of this form of
organic enrichment are severe but limited.
In the case of shellfish, farming of mussels has expanded in Spain, New Zealand, and
Ireland. The mussels are grown on suspended ropes, feeding on natural planktonic material. In
studies of mussel farming in the estuarine Ria de Arosa of northwest Spain, Tenore et al. (1982,
1985) have shown that mussels are a key species in determining ecosystem structure and
dynamics of the whole area. By intensive mariculture, man has replaced the zooplankton with
mussels as the dominant herbivores in the area. The major changes are:
1. The surface area of, and detritus from, the mussels support a dense epifaunal community,
which utilizes 90% of the mussel feces, and supplies food to demersal fish and crabs.
2. Epifaunal larvae, rather than copepods, dominate the zooplankton community.
3. Nutrient cycling by mussels dampens phytoplankton oscillations and contributes to high
seaweed production on ropes.
4. Heavy sedimentation of mussel deposits changes the sediment regime and lowers infauna
production.
5. Transport of particulate organics derived from mussel deposits from the farming area enhances
benthic biomass outside this area, and might support near-shore fisheries, especially for lake.
Tenore concluded that the raft culture of mussels affected food-chain patterns and
production in generally positive ways, although admitting that the infauna benthos near the farms
was typical of polluted conditions. Considering this study along with several others, we can
summarize in Fig. 6.14 the possible effects of mussel farming on the estuarine ecosystem. While
mussel rafts or ropes reroute the flow of energy or materials, they do not add any extra nutrients
to the ecosystem, unlike caged finfish fed on prepared food, and any changes involve processes
different from eutrophication caused by an enhanced nutrient supply.
Both finfish and shellfish culture will produce impacts on the indigenous fishes through
the alteration of normal habitats, impeding of water flow, and disruption of migration routes.
There is the introduction and/or displacement of nonnative stocks and the introduction of exotic
species. In cases where fishes such as salmonids have been reared especially for culture,
escapees from the genetic monoculture will produce an alteration in the genetic makeup and a
possible reduction in genetic fitness of the natural populations. There may be new and increased
disease introduction as well as parasite introduction/concentration. Finally, the removal of locally
or distant caught marine fishes for fish meal for feeding the farmed fishes will impact on those
stocks.
Given the importance of these areas for birds, aquaculture has the potential for impacts
on this use. There may be acute and chronic disturbance of breeding, feeding, and overwintering
areas and the disruption of the site (through the loss of land and water usage) as well as an influx
of opportunist birds feeding. The activities can lead to mortalities through the control by shooting
(which requires an exemption to bird protection Acts in certain countries) and there may be
accidental or deliberate trapping of birds (through the use of anti-predator nets etc.). These
adverse effects may be minimized by site considerations, gear improvement, and the use of
scaring devices or sacrificial food, and the adoption of a policy on shooting.
Figure 6.14 Summary of the possible effects of mussel farming. Note that some of the effects are
contradictory, and not all effects will be seen at one site. (From Gowen et al. 1988.)
Aquaculture requires the addition of chemicals and these may be given as enteric
treatments (by mouth, e.g., with the fishfood) or by immersion treatments. These include food
additives such as vitamins, mineral mixes, and pigments, the latter are required in the case of
salmonids in order to produce pink flesh, which is acquired in the wild by eating a crustacean
diet not used in farming. Anaesthetics and narcotizing agents are required to minimize stress of
the fish during handling and vaccines and other therapeutants are administered for health.
Disinfectants and antibiotics will be used to minimize microbial effects and, given the increased
possibility of external parasite transfer when large numbers of fishes are confined, pesticides
(such as Nuvan/Aquaguard (dichlorvos based) and ivermectin) are used to combat infestation of
fish lice. Finally, antifouling agents/ treatments (such as Cu, TBT, bitumen, and slippaints) may
be used to control macrofouling of the gear.
Aquaculture thus has the potential for impacts at different biological levels. Management
strategies may be employed to reduce or remove the effects of aquaculture. The developer may
collect the excess solids by funnel or suction system although decomposition, a release of
nutrients, and poor water quality can still occur. Pumping or bed trawling to aid breakdown could
disperse the wastes. Good siting principles will minimize the effects or the cages may be moved
at intervals, with those intervals being judged by the degree of bioturbation under the cages,
hence using the assimilative capacity of the area. Feeding techniques using low-wastage feed and
hand-feeding, coupled with the type of food (slow sinking, and with a balance controlled to
reduce NH3 input) will reduce the effects. The use of good practice such as the removal and
destruction of mortalities, rather than into the water column, and the use of biological control for
pests (e.g. lice control using goldsinny wrasse) will minimize biological effects. In addition,
fishfarmers and environmental protection agencies determine the potential effects using
numerical modeling (based on volume of stock, water flow, depth), coupled with self-monitoring
for the health of sediments and benthos. Sampling and/or photography then achieve the
assessment. Finally, the use of polyculture, as a mixture of fin- and shell-fish farming, or the use
of ranching as compared to cages can be a wiser use of the environment.
