In 'The Complete Book of the Roach' I describe how freshwater fish depend on the aquatic ecosystems they inhabit for food, flight and reproduction. These same three fundamentals apply to all life stages, from spawning through egg development and early life stages, for fry and growing juveniles right though to mature adults.
The first free-swimming life stages of freshwater fish depend on a diversity of food sources from individual algal cells through a succession of tiny to progressively larger invertebrates to enable them to grow.
Some, such as pike and perch, turn into dedicated predators, feeding on protein-rich invertebrate and vertebrate food sources including other fish. Others, such as roach, have an omnivorous diet responding to whatever plant or animal food is available with the turn of the seasons, even resorting to opportunistic predation on fish fry when presented with such easy pickings.
A balanced fishery
A diversity of spawning substrates is essential for the range of species constituting a balanced fishery. Many, such as roach, rudd and bream, shoal-spawn on available submerged and emergent vegetation. Tench and carp rely on shallow, weeded margins that warm rapidly, without which spawning may be unsuccessful or indeed may not occur.
Then there are species such as perch and zander that lay their eggs on hard submerged substrates such as woody or rocky debris including tree roots and overhanging branches. Gravel-spawning species – trout and barbel as well as chub and dace – tend on the whole to be less common in canals due to an absence of the flushed gravels essential for spawning and the protection of maturing eggs.
Successful spawning – or not
So the range of habitats available in canals automatically prevents the successful spawning of some species. For many species, rapidly-warming shallows with only gentle water movement adjacent to spawning sites are essential to allow fry to drift in and enjoy the warmed, food-rich environment. Without this ‘kick-start' to growth, they may not grow on throughout their first summer to become robust enough to withstand autumnal and winter extremes, though many canals are protected from the strongest flows that can wash many underdeveloped juveniles out of rivers.
Early life stages of fish are far from fully developed, their fins, musculature and guts incomplete and their bodies generally no longer than an eyelash. So, in addition to places for finding ideal nursery environments rich in food, refuge from currents is also important (particularly during summer floods).
Places of refuge
All fish, from the tiniest fry to progressively larger individuals, need to find refuge from currents. However, they also need places of refuge from a bestiary of predators ranging from larger invertebrates through to other fish, kingfishers, herons and cormorants, otters and a wide range of other piscivorous or omnivorous animals. Refuge habitat then is a constant requirement, and for a number of purposes.
As man-made and often working environments, canals cannot always be expected to be the most diverse of aquatic habitats. Whether we consider spawning gravels, warming nursery shallows, tangles of dead wood or marginal silt bars, canals are clearly deficient in some of the habitat features necessary for the most diverse range of freshwater fish species. Indeed, the habitat needs of different species automatically favours or disadvantages them for life in canal environments.
Vegetation in canals
It is for this reason that vegetation in canals is so important. Different types of vegetation, from fully submerged plant species to those with floating leaves such as lilies, stands of reeds, rushes and similar emergent plants, and including underwater tree roots as well as boughs dipping into the water, serve the spawning requirements of some species though not others.
A mix of vegetation can also provide some nursery and nutritional requirements, both directly and through the invertebrates and other aquatic life that thrives on and within stands of water plants. This compensates to some degree for the lack of ‘harder' habitats such as bays, side bars, scoured beds and bends. Also, very importantly, the ‘soft' habitat provided by vegetation serves vital refuge functions. Fish of all life stages need somewhere to evade their predators, just as predators too often require cover from which to ambush their prey. Also, for those canals that receive flood water or connect with rivers, vegetation provides vital refuges from stronger flows which, as we have seen, can be so detrimental for smaller and more vulnerable life stages.
A vital natural resource
Retaining and promoting a diversity of vegetation in canals is therefore absolutely vital. It is the best form of habitat available in many of these otherwise habitat-poor environments, supporting food, flight and reproduction for different life stages of fish and being crucial to the wellbeing and survival of many fish species. Think of vegetation in canals as a vital natural resource, and treat it as such if you want a healthy and resilient fishery.