Climate Change 2. - Where are the sandeels?

Over the summer I spent two weeks as a residential volunteer on the RSPB reserve at South Stack, Holyhead. During that time, I lost count of how many times I was asked 'Where are the puffins?'. Only once, however, was I asked 'Where are all the sandeels, then?'.

Until mid-to-late July, South Stack cliffs host to around 10,000 breeding birds; mostly guillemots, but approximately 1,500 razorbills too, along with a handful of other birds such as puffins, kittiwakes and fulmar.

Who are you calling a penguin?! Razorbills (Alca torda) on South Stack Cliffs. The guillemots and razorbills spend their lives at sea, and only venture onto land to breed. 

A large part of the diet of these seabirds during the breeding consists of sandeels; several species of slim, eel-like fish that occur in shoals that are important in linking together many trophic levels of the marine ecosystem. The primary species fed upon by bird colonies in the North Sea are the adult lesser sandeels (Ammodytes marinus) which, like all sandeels, burrow into soft sandy substrates with which they are closely associated. Their distribution is therefore restricted to areas in which such substrates are found, and as a result these fish are unable expand their ranges into deeper waters to deal with the threats they may face (Scottish Gov, 2010).

A reduction in the sandeels available with not only effect species of seabirds -Kittiwakes in particular-, but also marine mammals such as harbour porpoises (MacLeod et al, 2007) and up to 10 species of predatory fish (van Deurs et al, 2009). Sadly, sandeel populations have been declining in number, and this in turn has been linked to a reduction in sea-bird populations which struggle to feed themselves and their young. Between 1986 and 2011, the number of seabirds breeding around Scotland decreased by 53%, and the decline is believed to be due to a drop in sandeel availability (Herald Scotland, 2012).

So, where are all the sandeels going?

The population decline of the sandeels has been linked to two anthropogenic causes:

• Climate change, resulting in a warming of the UK coastal waters since 1980 of up 1°C per decade (Wanless et al, 2010).
•  Over-fishing for commercial purposes, such as for fish-meal feed and fertiliser.

Sandeel population decline has been correlated with rising sea temperatures by several authors (Wanless et al, 2010) (Heath et al. 2012). As the seas warm, the cold-water zooplankton that the sandeels feed upon are replaced by less nutritious warm-water varieties (Wanless et al, 2010). Though the cold-water species are still found in cooler, deeper waters, the habitat restriction of the sandeels means they cannot follow the cold-water zooplankton as they migrate (Heath et al. 2012). As a result, the sandeel populations experience a decline.

Rising sea levels are also thought to be posing a threat to the sandeels, though overall the effects of anthropogenic climate change on Ammodytes is not fully understood, and research is still ongoing as a result.

Whilst over-fishing is not thought to be the primary cause of sandeel decline, it makes it harder for the climate-hit sandeels to fight back. In turn, this affects seabirds; Sandeel exploitation has been linked to declining kittiwake colonies near to fishing grounds.

Plans were announced in November 2012 to halt the decline in seabird numbers (Scottish Seabird Centre). One of these plans involved the inclusion of sandeel habitats in the Scottish Government's Marine Scotland agency's network of Marine Protected Areas where there are imposed fishing limits. It is hoped that these fishing limits will halt the decline in sandeel numbers, which in turn is anticipated to stem the decline in seabird populations.

Bird Friendly Tuna?

Many albatross and petrel deaths attributed to pelagic long-line fishing for tuna and similar species in the southern hemisphere. In a study of seabird deaths due to pelagic longline fishing in the southern Atlantic Ocean between 2004 and 2008, Yeh et al. (2012) estimated that between 3446 and 6083 birds were killed per year. These deaths lead to many breeding-site populations reducing in number (Roberston et al, 2013).

A shoal of Skipjack Tuna (Katsuwonas pelamis) - image
from Wikipedia Commons.

There are ways to reduce mortality, such as setting lines at night, but this method -and several others- may mean that industries are unable to set their lines at times that optimise catch rates.

Robertson et al. (2013) suggest that focusing on branch-line weighting may be a way of reducing mortality rates of seabirds in the Australian fishing industry. Weights are not a new innovation in longline fishing, but Robertson et al. (2013) carried out experiments that differed from previous weighting methods. In their tests, weights were positioned differently upon the lines; in the first test, a 120g weight was placed ≤ 2m from the hook, and in the second test a 40g weight was placed directly at the hook. The purpose weighting the lines is to reduce time it takes for the hook and its bait to sink, thus reducing risks to seabirds by reducing the exposure of bait at the surface where birds may become caught.

Concern is often expressed that weights reduce catch, but Robertson et al. (2013) found no reduction in catch rates of Yellow-fin tuna; rather, catch rate correlated with bait type. For the other species caught, neither weighting or bait type seemed to have an effect on catch rates.

Though the heaver weighting method had faster sink rates, the authors acknowledged the implications of the extra weight in the gear bins. For this reason, they suggest using the lighter weighting method. They also indicate concerns for the loss of lead weights through shark bite-offs. Though not so much an economic problem, there is some concern that lead weights will become deposited on the seabed, or that they could be swallowed by sharks which may be later caught for human consumption. A way round this dilemma would be the replacement of lead with another substance.

Robertson et al. (2013) also state that loss from bite-offs may be minimised by placing weights on shorter leads; having the weights nearer hook is also safer for members of the fishing crew, as their experiments show such placement to reduce 'fly back' of the weights when hooks are bitten off. Other benefits they raise are reduced labour times for fishing crews due to less tangling and less like breakage (and therefore less repairs) of lines, as well easier line deployment.

New weighting regimes therefore seem like a win-win situation for both seabirds and the fishing industry. I just don't think the tuna will be all that happy...