Where have all the fishes gone?

An Argo float, during a NASA study of ocean temperature measurements. Credit: An Argo float, during a NASA study of ocean temperature measurements. Credit: Argo program, Germany/Ifremer

Menakhem Ben-Yami looks at the effect warming sea temperatures has on fish species.

During the last decade or so, fishermen - both professional and amateur - have been aware that while they continued to fish in their regular areas and times, the composition of their catches was changing. Species they had to travel south to fish for now frequented their home fishing grounds, while their regular fish species sort of moved away - in fact, northward in the northern hemisphere and southwards in the southern one, sticking to their customary temperature range.

According to Professor Axel Timmermann of the University of Hawaii, global ocean temperatures in 2014 were the highest on the record. In 2013, exceptionally warm water expanded throughout the Pacific Ocean, while in the Atlantic, sea-surface temperatures off the NE USA reached 150-year-record levels of about 14°C in 2012 - much higher than the 12.4°C average of the last 30 years, and kept rising fast.

It appears, claim Scripps Institute's scientists, that since the 1870s until 2012 the average global sea-surface temperature increased by 0.6°C (1.1°F), and oceans have been warming for some 100 years, while storing excess heat. In the north Pacific, exceptionally warm water extends now from Papua New Guinea to the Gulf of Alaska. On the top of it, the rate of this warming is now faster than ever recorded.

A climate researcher, Dr Paul Durack, is claiming that oceans' warming has been underestimated by over 50%, and that the build-up of oceans' heat represents the main cause for the global warming. So, how is all this is affecting the movements, distribution and behaviour of various fish species?

Roughly, marine organisms can be divided into two main groups. Those that can’t tolerate wide temperature swings are called stenothermal. Those that can are called eurythermal. The practical difference in view of the global oceans' warming consists in the migrations of mobile stenothermal species poleward and/or into the deeper waters where the temperature is lower, while the eurythermal ones stick around their native grounds.

Redistribution of marine species
According to Dr William Cheung and his colleagues at UBC, warm-water species are increasingly dominant in North Atlantic catches. Subtropical and warmer-water species, such as, for example, Atlantic croaker started appearing in fish yields taken up north. Salmon have expanded further north to such a degree that local, native fishing people haven't even had a name for them.

In 2009, NOAA researchers reported that over the last 40 years about half of 36 fish stocks in the northwestern Atlantic started a northwards shift, or migrated to deeper zones farther offshore. Thus, some species may become inaccessible to any local surface and amateur fisheries.

Altogether, ecosystems affected by shifting temperatures entered a sort of a continual havoc. This is because each marine ecosystem represents a complex structure in which change in a single parameter per force affects all its components.

Virtually, all fish rely on planktonic food at some stage of their life. Many species have evolved strategies to synchronise reproduction with seasonal pulses of zooplankton abundance.  Therefore, temperature changes may trigger major and complex distributional shifts not only of the basic marine organisms such as phyto- or zooplankton. For example, Kevin Friedland, of the U.S. National Marine Fisheries Service, says that shift of zooplankton along the US northeast continental shelf has affected summer and winter Atlantic cod spawning. Where zooplankton populations have declined there was fewer cod and vice-versa.

Obviously, shifts of populations of marine organisms aren't the single effect of sea warming, for in any eco­system, any change in abundance or whereabouts of one species must to this or another degree affect others, whether its prey or its predator, or both, as well as its competitor. When the water heats up, it would be a wonder if all the various components in an ecosystem respond in such accord that it maintains its former, however vacillating balance.

The global tuna catch might be affected by a drop in phyto- and zooplankton abundance in the warming Indian Ocean. About 20% of commercially sold tuna comes from the region, where it spawns and where its larval and post-larval species feed and grow.

A similar story seems to be unfolding below the equator in the Indian Ocean, says Dr Roxy M. Koll, a scientist at the Centre for Climate Change Research in Pune, India. Dr Koll estimates that warming water temperatures have reduced phytoplankton abundance by some 20%. The western Indian Ocean being responsible for approximately 20% of the global tuna catch, ecosystem changes due to sea warming in the Indian Ocean could potentially have significant effects on both the global tuna catch and the range of tuna fishing operations.

Warming and fish physiology
But how does temperature really affect fish physiology? Most fish are ectothermic, meaning they can’t regulate their internal temperature. To a great extent, that makes them slaves to their environment.

A fish residing outside of its thermal comfort zone might experience various physiological strains. Too-cold water might impair its proper digestion, while excessively warm water can lead to increased body temperature and energy requirements, which results in the need to consume more food to stay healthy.

Warming is also not good for dissolved oxygen levels in sea water. The cooler the water, the higher the levels of dissolved oxygen and vice-versa. Oxygen requirements vary from species to species, in general, however, fish prefer waters with more dissolved oxygen available.

In certain regions of the eastern tropical Atlantic and Pacific oceans, water with very low dissolved oxygen levels occurs in fairly close proximity to the surface. These ‘oxygen-minimum zones’ most often occur where an upwelling pushes cold nutrient water from the depths to the water’s surface, creating areas of high primary productivity. Such food abundance attracts fish of all sizes. Not all of this plankton is consumed, it dies and sinks – food for bacterial processes divesting layers of the water column of dissolved oxygen. All those warming-associated processes eventually cause many fish species to move along with their preferred water masses and away from their traditional grounds.



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