Sea trout, Salmo trutta, are found in rivers and coastal seawaters in many regions of the world. Sea trout lay their eggs in rivers and the young fish remain in this freshwater environment. After living for 1–8 years in freshwater rivers, sea trout migrate to the sea. Adult sea trout feed in coastal seawater regions, usually no more than 80 km from their home river. Fish farms that produce Atlantic salmon, Salmo salar, may be located in the coastal seawater regions where wild sea trout are also found. Fig. 2.1 shows one cage at a fish farm. [Figure 2.1] AIR SEAWATER net to stop birds fish cage farmed Atlantic salmon wild sea trout not to scale Fig. 2.1 The increase of fish farming is thought to have caused more infections by salmon lice, Lepeophtheirus salmonis, in wild fish such as sea trout. When sea trout enter coastal seawater regions, they can become infected with salmon lice. Salmon lice larvae live in seawater and can move freely between fish cages and open seawater. Salmon lice attach to the skin of sea trout and feed on their skin and blood. The resulting tissue damage can sometimes cause the death of sea trout. Fig. 2.2 shows a sea trout infected with salmon lice. [Figure 2.2] body of sea trout salmon lice Fig. 2.2

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

1. [3] 1 idea of: add lice to container and counting lice (on trout) at death or add lice to trout (in a container) and counting lice (on trout) at death or capture trout with lice attached and count lice (on trout) at death or trout(s) with (known) different number of lice attached (in separate containers) and see which die ; 2 idea of repeat with container (of trout) / trout (in a container) with no lice present ; 3 named environmental condition in container maintained as constant (for, experimental / control / both, conditions) or named feature of trout / lice maintained as constant (for, experimental / control / both, conditions);
2. [1] any one from: location of, boat / sampling site ; (electric) current ; time of year ;
3. [1] any one from: idea that young / small, trout will not be caught / AW ; ora idea that there will be more lice on the fish, caught / AW (as they are older / larger) ; ora
4. [2] allows for valid comparison; (because) number (of sea trout), sampled / AW, varies ;
5. [1] paired data quote / manipulated data (to show year 1 to 2 increase); Data quotes: mean % with ≥ year 13 attached lice 1 2.6 2 14.9 more / higher 12.3 times increased x5.7
6. [2] any two from: 1 no difference between year 1 and year 2 of production cycle in 2004–2005 ; 2 number of sea trout sampled was small; 3 (data from) only, 5 production cycles / 10 years ; 4 no statistical test carried out ;
7. [1] there is no, relationship / correlation, between the number of lice (on trout) and the distance from the (nearest) fish farm ;
8. [1] (Ireland) significant / yes / ✓ / AW and (Scotland) not significant / no / × / AW ;
9. [1] there is a negative correlation or as distance from (nearest) fish farm increases, (mean) number of lice (per trout) decreases ; ora
10. [2] Max one from MP1–3 reasons for 95% CI: 1 95%, probability / confidence that/chance that, the, true / actual / population / parametric, mean lies within, this range / these limits or 2 to test whether the means (for number lice per trout at different distances from fish farms) are (significantly) different or to see / test, whether the, error bars / 95% CI / they, overlap or 3 to get a measure of how close the, calculated / AW, mean is likely to be to the, true / AW, mean or shorter, error bars / 95% CI, show true mean is likely to be closer to the calculated mean or longer, error bars / 95% CI, show true mean is likely to be further from the calculated mean ; One mark for exemplification: Exemplification of MP 2 or 3 from Fig. 2.3: MP2 Exemplification: idea that no / less overlap of (95% CI) error bars for Ireland, so (difference), is / may be, significant or idea that overlap of (95% CI) error bars for, Scotland / Ireland, so (difference), is not / may not be, significant; I direct comparison between Ireland with Scotland MP3 Exemplification: idea that the, shorter / narrower, error bars for Ireland the closer the calculated mean is (likely to be) to the true mean or idea that the, larger / longer, error bars for Scotland, the further the calculated mean is (likely to be) to the true mean or shorter / narrower, error bars / confidence limits, with increasing distance (to nearest fish farm) in, Ireland / Scotland, shows the calculated mean is, getting / AW, closer to the true mean; I direct comparison between Ireland with Scotland