‘THE ZANDER IN PERSPECTIVE’
By
Linfield and Rickards
The newly hatched zander larvae are 4mm long, and commence feeding within two days, at a length of 5mm to 6mm long. The fry become pelagic and feed on zooplankton for the first few months of life. They may become piscivorous at approximately 2.5cm long, but fish are of minor importance to the diet until they are about 10cm long.
British zander don’t appear to have any prey type preference, and availability seems to be the only governing factor in prey choice. While the diet of adult zander consists mostly of fish they do eat significant quantities of invertebrates, and scavenging also occurs.
Estimates of percentage body weight of prey consumed daily by adult zander vary from 0.5% during periods of slow growth, to 5.5% at peak growth rates, with an annual intake of 200-250%.
It has been observed that feeding predominantly takes place at night for most of the year. Daytime feeding occurs extensively in coloured water conditions with poor light penetration. This is explainable by the retinal structure of the zander, which causes the fish to achieve optimal visual performance in low light conditions.
The smaller mouth of zander compared to pike results in the size of prey taken being smaller than that by pike, and there have been reports that prey fish are not normally longer than 150mm, however packs of zander have been observed to attack much larger prey of up to 3lb which then drift into the margins and die. It is assumed that these fish are then scavenged later.
In Russia weights of up to 20 kilos and lengths of up to 1.3 metres long have been recorded. In optimal growth conditions the species therefore has a greater growth potential than that usually realized in western European water.
While the maximum age for zander is usually no more than 13 to 15 years, they have been recorded at up to 20 years, and the walleye at up to 26 years.
British zander are generally mature at 3 to 4 years old, but in colder areas maturity may be delayed by up to 5 years. Growth rate also influences the age at which maturity is reached, with rapid growth leading to earlier maturity than slow growth rates.
During reproduction female zander generally produce about 200,000 eggs per kilo of body weight, and in this respect appears to have a higher reproduction potential than pike.
The time of spawning is dependent on water temperature usually occurring in spring, at temperatures of approximately 12 degrees centigrade, although variations of between 8 to 15 degrees centigrade have been recorded.
Captures of both fry and ripe females suggest that April or May is the favoured spawning time in the UK. The eggs, which are whitish-yellow in colour and approximately 1.3mm in diameter, are laid in clumps amongst weed roots exposed in a ‘nest’ prepared by the male. The eggs take approximately 10 – 15 days to hatch, during which time they are actively protected by both parents. The mortality rate of fry during the first year is very high, giving rise to a relatively small recruitment to the population. Fecundity is not therefore a reliable guide to general reproductive capacity but the large number of eggs laid appears from experience of the species in Fenland to enable very strong year classes to be formed in years when environmental factors are favourable for fry survival. It has not yet been established whether zander have similar habits to walleye in showing homing behaviour to return to the same spawning areas each year.
It has been estimated that the population of zander in the Relief Channel in 1966 and 1967 stood at 20,000. If the average weight of these fish was 400g, the Channel at that time therefore supported at least an additional 8 tonnes of predator biomass than in early 1963, the true value probably being somewhat higher. Taking the estimated annual food ration of 200-250% of body weight this rapidly increasing stock of predators would have eaten in excess of 16-20 tonnes of prey fish per year in 1966-1967, with the quantity rising each year until the species reached a population peak.
In the early 1960’s a notable feature of the Relief Channel was the very substantial head of small fish, including bleak, dace, rudd, roach, gudgeon, silver bream, ruffe, and perch, which were observed many times in vast shoals. These fish were thought to have been predated on extensively by the increasing zander population, and by 1969 their numbers were greatly reduced. The circumstantial evidence pointing to predation by zander as the major cause of the marked decline in stocks of small fish is strong. Further evidence is provided by observations at Rosswell Pits, near Ely, where the same pattern of a marked decline in the previously numerous small fish population followed the arrival of zander from a Relief Channel connection with the Ely Great Ouse in 1970. Coincidental with the reduction in numbers of small fish in the Relief Channel fishery was the apparent increase suggested by anglers catches of much larger fish in the water, notably shoals of large bream, and it is thought that the structure of the cyprinid populations changed such that a greater proportion of their biomass was present as large fish relatively safe from predation.
It is suggested that the Zander occupies a slightly different ecological niche that it’s main competitors. The diet of adult pike generally includes less invertebrate and larger prey fish than the diet of zander of equivalent size. Thus once the initial rapid increase in numbers of zander in a newly colonized fishery has passed its peak the equilibrium between the predatory species which is re-established might be expected, other factors being equal to involve a higher total density of predators than was previously present. The night time feeding habits of zander, when combined with the largely day time predatory activity of pike, might also be expected to cause a greater increase in predator biomass because of the attendant increase to 24 hour per day predation.
The overall effect on the quality of an angling fishery which results from the addition of zander to the fish fauna depends on a number of factors, particularly:
(a) The sizes and species of fish anglers want to catch.
(b) The previous density of other fish species.
(c) The levels of recruitment and production of supporting prey species.
The first of the above factors varies between anglers, and the remaining factors vary between fisheries and between years within a fishery. The predicted effect of zander introduction on different types of fishery may be illustrated by taking two opposite examples.
Type A:
Very large fishery with a consistently high recruitment and production of cyprinid species. Relatively low angling pressure per unit area.
Type B:
Small fishery with a consistently poor recruitment and production of cyprinid species. Very high angling pressure per unit area.
Type A may be likened to the Great Ouse Relief Channel, where the nature of changes, from general observation and anglers catches has been described. Those anglers who particularly enjoyed fishing for zander or large cyprinids and were not concerned by fishless days tended to take the view that the fishery benefited from the zander’s introduction. Conversely those who liked to catch at least small fish consistently, and those who were primarily concerned with the pike tended to take the view that the quality of the fishery deteriorated. In general the colonization of Type A fisheries by the zander can be expected to cause changes in the nature of the fishery, which on balance could be said to be neither good nor bad, but which cause it merely to be different.
Type B may be likened to the small Essex Rivers under threat from colonization by zander. In waters not able to produce good recruitment of cyprinids, a large piscivorous species that is also able to turn to invertebrates for food might be expected, should factors prove favourable for its own reproduction, to become dominant in the population. In waters where such predator – prey imbalances already exist with the pike, the addition of zander will clearly be viewed as undesirable, except by persons primarily interested in zander fishing.
The effects of zander on a newly colonized fishery are in practise more difficult to predict than the above might suggest because the characteristics of most potentially suitable waters fall between the two types illustrated. Furthermore the levels of recruitment of cyprinid species in the UK tend to fluctuate between years according to both climatic and other factors, and when two more ‘good’ or ‘bad’ years for a particular species occur together major cycles of prey imbalance can occur. It might therefore be anticipated that the prescence of an increased biomass of predators dependent on a high level of recruitment of prey fish will make cyprinid cycles of abundance more pronounced through their effects on the background stock during years when cyprinid recruitment happens to be poor. In these circumstances recovery of the cyprinids from a period of poor recruitment might, if background stocks become severely depleted be delayed by the predatory pressure until the predator biomass adjusts to the lowered level of prey availability, thus spreading the cycle abundance over a greater number of years. For the moment insufficient data exists on the status of either the zander or species in colonized rivers/drainage channel fisheries in Anglia to ascertain with certainty what detailed long term effects the species has produced.
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