Climate Influences on Deep Sea Populations

The data gathered to conduct this study shows that strong downslope currents associated with intense cascading events displace the population of this deep-sea living resource towards greater depths, producing a temporary fishery collapse.

“The detailed analysis of the major dense shelf water cascade occurring in winter 2005 determined that the high near-bottom current velocities associated to this oceanographic phenomenon, displaced the population of this deep-sea shrimp from their fishing grounds,” explains Pere Puig.
This is the reason that explains the recurrent (every 6-11 years) disappearance of this priced living resource, a mystery that has been finally solved.

Despite this first negative effect, dense shelf water cascades also cause an enhancement of the recruitment process of this species and an increase of its total landings during the following years. 

“Far away from the fishing grounds, at depths greater than 1500 m, is where the larvae of this deep-sea shrimp settles to the seafloor. The large transport of nutritive particles brought by cascading waters to deep-sea regions assures a high larval survival of this species, and two years after, when the juvenile shrimp population migrates to shallower depths, where the fishing grounds are located, the amount of shrimp brought back by the trawlers increase enormously,” says Joan B. Company.

This behavior suggests that dense shelf water cascades, a climate-induced phenomenon originated during cold and windy winters, could control the long-term biological processes of deep-sea living resources worldwide. “This process was largely unknown until now, and adds more data demystifying the idea that the deep-sea regions and the surface coastal waters are two isolated ecosystems” adds the marine biologist. Thanks to the data provided in this study, it could be monitored the oceanographic conditions that trigger the disappearance of the red shrimp and predict when they it will return to the fishing grounds located at depth of 500 to 900 m. This information will allow implementing management plans to conduct a sustainable exploitation of this living resource.

The researchers revised historical landings and oceanographic databases since late 70’s and could determine that this recurrent phenomenon repeatedly reverse the general decreasing trend of Aristeus antennatus landings, presumably linked to overexploitation.

“The red shrimp, as many other deep-sea living organisms, are very sensitive to fishing overexploitation since their reproductive cycles occur in late ages, and an intense fishery pressure reduce the number of individuals that can reach their maturity,” explains Company.

These new findings resolve the paradox of a long-overexploited fishery that has not collapsed after 70 years of intense deep-sea trawling. These new results will have a high socio-economic impact; since this species is the most valuable deep-sea living resource in the Mediterranean Sea (it can cost as much as €230 a kilogram).

The authors extrapolate their findings to a global fishery scenario, and hypothesize that dense shelf water cascading sites identified worldwide could be considered as regions favorable for deep-sea demersal fisheries, in a similar way the upwelling zones are considered favorable regions for pelagic fisheries.

This paper is particularly timely, since these new results will be of special relevance to the current debate on the shift from shelf to deep-sea fisheries.

Dense shelf water cascades

Dense shelf water cascading is a global climate-driven oceanographic phenomenon common not only on high latitude continental margins, but also on mid latitude and tropical margins. “The Gulf of Lions is one of the three known regions in the Mediterranean where this phenomenon occurs almost every year. Winter heat losses and evaporation induced by persistent, cold and dry northerly winds cause densification and mixing of coastal waters, which sink, overflow the shelf edge, and cascade downslope at high current speeds, up to 1 m/s,” explains Pere Puig.

Through this phenomenon, dense shelf waters carrying large quantities of dissolved and particulate matter are rapidly advected hundreds of meters deep, mainly through submarine canyons, acting as a significant natural carbon sequestration and as a deep-sea ecosystem fuelling mechanism. This climate-driven process, which is subjected to the intensity and duration of northerly winds and the amount of freshwater inputs to the Gulf of Lions, occurs annually.