ii) The risk of biomagnification increases with numerical trophic value, The saithe seems most susceptible to biomagnification, (the increase in concentration of a toxic substance as you increase trophic levels), due to its role as a tertiary consumer at the later stages of its life \cite{Homrum_2012} - as opposed to the whiting and the sole, at maturity secondary consumers make up a greater proportion of its dietary biomass, increasing the magnification of heavy metals within its tissues.

c) Identify a non-fish species (exc. Humans) that is a known major consumer of at least one of your 8 focal species that could be used as a potential ecological indicator of the health of the N Sea fishery (ensure you cite your sources). What functional traits of this species would make it a useful indicator?

 The common stingray is a consumer of young herring in the North Sea, and can act as a direct competitor for larger herring \cite{Heath_2012}. It has been previously suggested that the spawning biomass and mortality of the pelagic stingray is a potential ecological indicator for the mortality and spawning biomasss of the herring \cite{Kabuta_2003}.   The common stingray is easy to identify from other ray species due to its distinctive tail, they also have a predictable feeding pattern making catching easier to accomplish high trophic level (3.5-3.8), and so monitoring populations of stingray is considerably easier than monitoring herring stock directly \cite{Coll_2016,Kleisner_2015}.

d) Describe how size-based measures might be developed as ecosystem-level biomonitoring indicators of the health of the N Sea fishery as a whole

 Many life history traits of organisms are correlated with body size, and trophic roles are intrinsically linked to overall size (and trophic roles can change over the lifetime of an organism), hence size-based measure can theoretically be used to examine food-web health and changing life history traits of individuals \cite{SHIN_2005}. Fishing directly decreases the community abundance and biomass, which indirectly affects population selection processes (favouring slow growth, early maturation, smaller overall body size) \cite{F__2002}. Monitoring key factors such as mean biomass of a population, mean length at maturity and size diversity can indicate shifts in ecosystem roles in the North Sea.

Q5. Atlantic salmon fish farms in Scotland rely heavily on fishmeal derived from sandeels – why has this been criticized on ecological grounds? 

Sandeels are a critical part of the north sea foodweb, with many predators including seals, seabirds and many predatory fish. Increases in sandeel stock are associated with significant decrease in the stocks of cod, haddock & herring \cite{Sherman_1981}, and is predicted to be linked to declines in seabirds and seals as well. Closures of sandeel fisheries has been linked to increases in some populations of top level seabird predators,  and fear over the potential link between overfishing of sandeels and nearby declines of seabirds has led some sandeels fisheries to shut down \cite{GREENSTREET_2006,Daunt_2008}.  The effect of intensive sand eel farming on sea bird populations has been debated. One such paper, funded by the International Fishmeal and Oil Manufacturers Association, indicated that the population declines associated with intensive sandeel farming are minor, and limited to a proportionally small number of sea birds \cite{Furness_2002}.