1. NATO ASI, October 2003William Silvert Types of Impact The ways in which fish farms can affect their environment

2. 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.

3. 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.

4. 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

5. 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.

6. 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.

7. 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?

8. 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.

9. 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!

10. 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.

11. 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.

12. 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.

13. 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.

14. 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

15. 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.

16. 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.

17. 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.

18. 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!