Presentation on theme: "NATO ASI, October 2003William Silvert Types of Impact The ways in which fish farms can affect their environment."— Presentation transcript:
NATO ASI, October 2003William Silvert Types of Impact The ways in which fish farms can affect their environment
NATO ASI, October 2003 Oxygen Depletion Fish and shellfish consume oxygen. If the density of farmed animals is too great, they can suffocate. Oxygen levels are current-dependent, and oxygen stress is most likely at slack tide. Oxygen can also be depleted by algal blooms and other events.
NATO ASI, October 2003 BCL BCL (Benthic Carbon Loading) is the most commonly modelled impact of fish farms. Both fish and shellfish release faeces which fall to the bottom and affect productivity, oxygen consumption, and community structure. Finfish farms also lose feed to the bottom.
NATO ASI, October 2003 The P-R Model Pearson and Rosenberg (1987) proposed a model – really a description - of what happens to the seabed under progressively greater carbon loading: Increased productivity Loss of species diversity Dominance by Capitellid worms Overwhelming the biota Anoxia & outgassing
NATO ASI, October 2003 Beggiatoa Mats, etc. The Pearson-Rosenberg model seems to describe a lot of fish farms. Under heavy carbon loading we see several symptoms, one of which is the growth of bacterial mats, usually of Beggiatoa. This can be accompanied by anoxia and outgassing of sulfide gasses.
NATO ASI, October 2003 Modelling BCL Usually BCL is modelled by following the track of the faeces and feed pellets (for finfish) as they fall. This involves using Stoke’s Law for the settling speed of the particles and allowing for the spread by tidal and other currents. The area where particles fall is called the “footprint” of the farm.
NATO ASI, October 2003 What is falling? Unfortunately it appears that the idea that settling particulates can be modelled in this wasy is questionable. Faeces from salmonids are often mucoid strings that drift through the water and can get caught in the structure of the cages. So how do they get to the bottom?
NATO ASI, October 2003 Flocculation As particles fall through the water column they interact with each other, and can adhere to each other. Ths process, known as flocculation, has a major effect on settling speed and on the properties of the particles when they encounter a surface, either the structure of the farm or the bottom.
NATO ASI, October 2003 Episodic settling One hypothesis is that the model of constantly settling particulates is only partially true, and settling is episodic. Material gets caught on the cages – not just faeces, but also mussels, epiphytes, etc., and gets shaken loose during storms or periods of high currents. This would completely change settling!
NATO ASI, October 2003 Resuspension Another problem is that we don’t want to know where the BCL first reaches the bottom, we want to know where it ends up. Often it is resuspended and carried off by bed load transport or other mechanisms. In general it moves from erosional to depositional sites.
NATO ASI, October 2003 Recovery Once material reaches the bottom it is subject to geochemical and biological processes. These processes affect the rate of impact and in the long run determine how long it will take an impacted seabed to recover after a farm or cage is removed.
NATO ASI, October 2003 Nutrients Nitrogen, phosphorus and other nutrients are released by fish farms. These occur mainly as dissolved compounds which are transported with water masses. Modelling nutrient dynamics is mainly a matter for physical oceanograhers in conjunction with phytoplanktologists.
NATO ASI, October 2003 Phytoplankton Nutrients can increase primary production if algal growth is nutrient limited. This is generally desirable, but too much primary production can lead to sinking, decomposition, and hypoxia. If additional nutrients stimulate harmful algal blooms, the consequences can be very bad.
NATO ASI, October 2003 Interactions Farmed organisms can interact with wild stocks and other marine organisms. Some of these effects are: Use of pharmaceuticals Escapes and genetic interactions Trapped wild fish Disease transmission
NATO ASI, October 2003 Pharmaceuticals The use of antibiotics, sea lice treatments, and other pharmaceuticals is a major concern in the management of aquaculture. Pharmaceuticals that have beneficial effects on fish can be deadly to crustaceans in even trace amounts.
NATO ASI, October 2003 Escapes There is concern that farmed fish, which are usually specially bred, may escape and mingle with the gene pool of wild stocks. This is sometimes avoided by using sterile stocks. Dealing with escapes involves modelling the risk of infrequent events.
NATO ASI, October 2003 Trapped wild fish Sometimes small wild fish enter into a fish farm and grow too big to escape. This is seldom a serious problem, although it can be an annoyance to the farmer who is feeding fish of no commercial value.
NATO ASI, October 2003 Disease Any time we have dense concentrations of animals there is an increased risk of disease. Fish farms have massive disease problems, and contagion is increased when farms are too close. Epidemiology is one of the most interesting fields for modelling!