The Clyde is a remarkable marine environment, but beneath its surface a complex network of environmental changes and human activities is reshaping the conditions in which marine wildlife must survive.
The Firth of Clyde has undergone enormous changes over the past century. Once renowned for its herring fisheries and abundant commercial fish stocks, it is now an ecosystem shaped by a long history of industrial activity, intensive fishing, coastal development and increasing demands on its marine resources.
Yet the Clyde remains biologically productive, supporting a diverse range of marine life, including harbour porpoises, dolphins and occasional visiting whales.
Understanding how these animals respond to a changing environment is becoming increasingly important, particularly as the effects of different human activities begin to overlap.
A changing marine ecosystem
Scientific studies have documented substantial changes in the Clyde’s fish populations.
Research by Heath and Speirs (2011), subsequently considered in the Scottish Government’s Clyde Ecosystem Review (2012), revealed that although the Clyde remained productive, the composition of its demersal fish community had changed dramatically. Large fish had become less common, with the community increasingly dominated by smaller species, particularly whiting.
For marine predators such as harbour porpoises, these changes matter. Their distribution and feeding behaviour are closely linked to prey availability, and changes in fish abundance, size and distribution can influence where and when porpoises forage.
The effects are not necessarily straightforward. An ecosystem dominated by smaller fish may still provide valuable feeding opportunities. However, changes in prey quality, abundance or accessibility could alter the energetic demands placed on predators.
Importantly, the Clyde’s fish community is not static. It continues to respond to fishing practices, environmental conditions and wider ecological processes.
The growing importance of underwater sound
For cetaceans, the underwater environment is also an acoustic landscape.
Harbour porpoises depend on high-frequency echolocation to navigate, locate prey and investigate their surroundings. Dolphins similarly rely on sound for communication, orientation and feeding.
The Clyde supports shipping, ferries, fishing vessels, recreational boating, aquaculture and naval activities. Construction, dredging and other industrial operations can also introduce underwater noise.
These sounds vary enormously in frequency, intensity and duration. Not all are equally harmful, and their effects depend on the hearing sensitivity of the species, the distance from the source and the characteristics of the sound.
Nevertheless, scientific research has established that underwater noise can affect marine mammals through behavioural disturbance, displacement from habitat, interference with acoustic activity and, at sufficiently high exposures, hearing injury.
Particular attention has been given to acoustic deterrent devices associated with fish farms. Although designed primarily to discourage seals, some devices can produce sounds capable of affecting other marine mammals.
Understanding the contribution of these different sources to the Clyde’s underwater soundscape is an important part of assessing habitat quality.
Fishing, entanglement and changing food webs
Fishing presents several different pressures on cetaceans.
Accidental entanglement in static fishing nets, known as bycatch, is a recognised cause of mortality in harbour porpoises. Bottom-set gillnets and tangle nets are of particular concern because animals can become trapped underwater and unable to surface to breathe.
Fishing can also influence cetaceans indirectly through changes in prey populations and seabed habitats.
This illustrates an important distinction: an activity does not have to cause direct injury to influence a marine mammal’s habitat or feeding opportunities.
Effective assessment therefore requires consideration of both direct mortality risks and the wider ecological consequences of fishing.
Pollution and the legacy of industry
The Clyde has a long industrial history, and although improvements in wastewater treatment and environmental regulation have reduced many historic sources of pollution, contaminants remain an important consideration.
Persistent chemicals can accumulate through marine food webs, eventually reaching predators such as harbour porpoises and dolphins.
Research involving stranded UK harbour porpoises has demonstrated associations between PCB contamination and reproductive failure, highlighting the potential for pollutants to affect marine mammal populations over long periods.
The health of individual animals can therefore provide valuable information about environmental conditions that are not immediately apparent from observations at sea.
Climate change adds another dimension
Sea temperatures around Scotland have increased, and the Clyde is no exception.
Warming waters may influence plankton production, fish distribution, seasonal feeding opportunities and the composition of marine communities.
For cetaceans, some of the most important consequences may arise indirectly through changes in their prey.
Climate change is therefore not simply another pressure acting alongside fishing, pollution or underwater noise. It has the potential to alter the ecological conditions under which all these other pressures operate.
The precise consequences for Clyde cetaceans remain uncertain, making continued monitoring particularly valuable.
When pressures overlap
Perhaps the greatest challenge is understanding what happens when several pressures affect the same animals, in the same places, over the same periods.
Consider a harbour porpoise feeding in one of the Clyde’s sea lochs.
The animal may be using an area where vessel traffic, aquaculture activity and fishing overlap. Its prey may be influenced by environmental change, while underwater noise could temporarily interrupt feeding or make parts of its habitat less suitable.
Individually, some of these disturbances may appear relatively minor. Together, however, they could increase the animal’s energetic demands or reduce the time available for successful feeding.
This is the principle of cumulative environmental impact.
Importantly, cumulative effects are not always simply the sum of individual pressures. Some may reinforce one another, while others operate independently or vary seasonally.
Neither does the presence of porpoises automatically demonstrate that an area is environmentally healthy. Animals may continue using valuable feeding grounds despite substantial disturbance, particularly where suitable alternative habitats are limited.
Understanding cumulative impacts therefore requires more than recording the presence of individual activities. It requires knowledge of where animals occur, how they use their habitat and how environmental pressures overlap in space