We all have our favourite types of environment and weather. Some love those warm, sunny days spent on a beach of golden sands. Some love those rainy days in the forest, when everything glistens with the raindrops. Some love nothing more than a cold crisp day in snowy mountains. We humans are lucky. We can not only survive but enjoy a wealth of different environmental conditions. Many other species are not so adaptive. In the oceans some creatures live in the seabed itself, others on top. Some may stay in the water column dominated by a particular type of habitat like a kelp forest, whilst others roam into a variety of different locations throughout their lives. Then there are the varying conditions of the ocean itself. Some areas are generally calm whilst others may experience a lot of movement. Salinity levels also vary, as does oxygen, as does temperature.
Actually temperature – as many a fisher will know - is a super important driver of species distribution. There are a few reasons for this. First, unlike us, most fish do not have the ability to control their own body temperature. Their internal body temperature reflects that of the environment they are in. The second primary reason relates to food. If the major food of a fish – be it plant (phytoplankton) or animal – changes its abundance (how many) or its distribution (where it is), then the fish may follow. The world's seas are getting warmer. On a global scale, measurements indicate that between 1971 and 2010 the upper 75 meters of the ocean has warmed by 0.11⁰C. Doesn’t sound like much but already numerous scientific studies as well data garnered from traditional ecological knowledge and those working in the oceans are pointing to shifts in some – but not all - species distributions. Pelagic fish – those who spend their lives in the water column, seems to be on the move more than benthic fish, who spend their lives on or near the sea floor.
Then there is location. The on a global scale the ocean surface may have warmed by 0.11⁰C, but it has not uniformly warmed by 0.11⁰C. Twenty major global warming hotspots – areas in which the ocean is warming much more rapidly than the average – have been identified in the world. One of those hotspots is the North Sea. The North Sea is, like all seas, home to pelagic fish… pelagic fish that aren’t just ecologically important, but commercially. Between 2000 and 2011 six such species - European sprat (Sprattus sprattus), Atlantic herring (Clupea harengus), Atlantic mackerel (Scomber scombrus), Atlantic horse mackerel (Trachurus trachurus), European pilchard (Sardina pilchardus) and European anchovy (Engraulis encrasicolus) made up an average 39.1% of the 8,965,617 tonnes of fish harvested each year from the Northeast Atlantic. These guys are also known as ‘forage fish’ and make up key positions in the Northeast Atlantic marine food web. And because of their fairly quick lifecycles, they are among those most likely to respond to changes in water temperature. It appears that they have done just that.
Ignasi Montero-Serra from the University of Bristol and University of Barcelona, and collaborators didn’t use and special monitoring techniques to uncover the changing distributions of these six key fish species. Instead they used data that was already available. To get a grasp of where species are – and indeed some key prey species, they used Fisheries-independent data from International Bottom Trawl Surveys collected between 1965 and 2012. Wait a minute… bottom trawl you say? These fish are pelagic right?! This is true, but the authors note that the data collected by these surveys are reliable indicators*. The UK Meteorological Office Hadley Centre’s Global Ocean Surface Temperature (GISST) databank was able to provide them with monthly mean sea surface temperatures for the same time, and the same areas the fish data came from. Actually this study claims to be the first to be carried out over such a long time scale, and such a large area – both important factors for assessing changes in species distributions. Put sea surface temperature, prey and the fish distribution data together, do some fancy calculations and you have information on how the six fish species distributions have changed over time – and how that correlates to any changes in average temperature and prey distribution.
Each species had its own unique distribution – and change in distribution over time. Prey distribution does matter, but more important is sea surface temperature itself. Generally, it seems that the sea surface temperature of the previous year had a strong influence on species occurrence. In effect this means temperature matters for reproduction and larval survival, and reproduction and larval survival matter for abundance and distribution. Warmer waters, the researchers note, can increase growth and metabolic rates of the early stages of fish, but it also means the larvae need to eat more – and thus are at greater risk of starvation (the risk of starvation – and indeed being eaten – is already very high for larval fish). More specifically, it seems that sardines, anchovies and pilchards in particular are moving increasingly more north, occupying the higher temperature environments that are now found there. For fishers in the south that target these guys, this shift is bad news. Not all species changed latitudinal distribution though. European sprat, Atlantic house mackerel, and Atlantic mackerel didn’t really shift (but did show changes in abundance with copepod blooms).
For more northerly fishers who may want to fish sardines, anchovies, and pilchards this shift may be good news. Take home message for the North Sea fishing community… make sure you – and your community as a whole – can adapt to these changing distributions. The source of your income may have moved on.
* For those who are interested, have a look at the paper for a link – which is just below - to the survey methods
The paper was published in the journal Global Change Biology and has been made open access by the authors (hurrah!). Have a read of it here http://dx.doi.org/10.1111/gcb.12747
This post also appeared in Scuba Travel’s Scuba News.