Types of Impact

1. Types of Impact The ways in which fish farms can affect their environment

2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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!