Wednesday, 11 March 2015

The herring

To better understand the orcas we also have to understand their environment and, specifically, what they are feeding upon. And in most places where we study them their main prey is an abundant silver fish: herring.

Photo by Filipa Samarra
Herring is the unsung hero of this ecosystem. It supports the whales; in groups the whales encircle schools of herring and using their tails they slap a bunch of herring that gets stunned allowing the whales to feed on it one by one. But not all the stunned herring is eaten by the whales, so when the whales are feeding some uneaten herring floats to the surface. Seagulls and fullmars are quick to catch these and take advantage of the whales' work.
Photo by Sara Tavares
Then there are the beautiful gannets. They wait until the whales have rounded up a nice ball of herring and then plunge the water in pursuit of some fish too. It is an incredible sound hearing a group of gannets simultaneously diving into the water. And we always wonder how they don't hit the whales and well...sometimes they do!
Photo by Sara Tavares
And of course there are also the sea eagles we see in the winter and the skuas we see in the summer. But it's not just birds, we also see white-beaked dolphins and seals in the area, which are probably also getting their fare share of the silver prey. And there's also all the unseen fish, like cod, preying on herring.
Photo by Leticiaà Legat
Photo by Miguel Neves
Photo by Marjoleine Roos
Photo by Sara Tavares
Although many different species can depend on herring, the herring itself is a tricky prey and this year we have seen how the herring distribution can quickly change. Up until 2014, there was a huge amount of herring overwintering in the fjords of Grundarfjörður and Kolgrafafjörður. But this year although the herring stock estimates indicate it is still quite a healthy stock, the amount of herring present in this area is much smaller. This is because the majority of the stock is spending the winter in the offshore waters further West. In fact the herring only started coming to this area in large numbers in 2006, following yet another shift in its distribution (see this study for more information). 

It is quite a mystery exactly why herring shifts its overwintering location. During winter, herring doesn't feed and it is mainly trying to avoid spending energy or being predated, and it often chooses coastal areas to do so. It is not possible to predict how long the herring will remain offshore or even what proportion of the stock will do so. It could be that in the winter of 2016, a large proportion of the stock once again travels to the fjords to overwinter, as it did in the last few years. 

Interestingly, it seems that it is during winter that the herring aggregates in higher numbers in relatively small areas. Throughout the rest of the year, the stock can be much more dispersed. And with the herring come the whales, so it is no surprise that it is in the winter months that we can see the largest aggregations of whales too. In fact, it is only in winter that some individuals can be seen around Iceland, as it seems that during summer they go elsewhere (see our post about matches to Scotland). So there is a lot that can be learnt about orcas by having the herring in accessible locations during winter, as only then can we follow the lives of some whales. 

So we keep our fingers crossed that this has been an odd winter, and that next year the herring return to familiar shores!