Winters Of Discontent by Donovan Kelley

Probably the worst thing that can happen to a young bass is a long cold spell in its first winter. Most recent winters have been mild, but cold spells can still occur. We had one in the 1995/96 winter and another in 1996/97, in each case of about 3 weeks duration. Fortunately bass seem able to survive spells of that length with impunity. In the early days of 0-group monitoring we were able to get a clear indication of how long they can endure them before they begin to die.

In the Taw nursery in N. Devon there is (or used to be: it is largely silted up now) an excellent sampling site where bass can (could) be caught throughout the winter. We were sampling it in the 1985/86 winter when we were hit by a quite severe cold spell. It lasted for 5 weeks with January and February temperatures well below normal. The previous summer had been a cool one, and the young bass had made poor growth. Our winter sampling, before and after the cold spell, showed that all the bass which had not reached a fork-length of 60 mm by the onset of the cold did not survive the spell. They were there before it began, gone after it finished, and never seen again. Clearly a 5-week spell was too much for them. In the Taw the loss was about 40% of the year’s stock; in the Tamar also 40%, and in the Camel – where growth had been even poorer – 100%: we never encountered any 1985 class juveniles in the Camel.

In the following winter there was a very intense cold spell in January. Temperatures in Cornwall were the lowest ever recorded in that month. But it only lasted for four days – and no casualties occurred, not even among the 23% which were under 60 mm at the onset of the cold. So duration rather than intensity was the key to possible mortality in spells. But it remained to be seen where the dividing line lay – how long the spell had to be before the under-60’s began to die; and perhaps whether it might be shorter for really intense cold.

For a really long (and severe) cold spell we have to go back to the Arctic winter of 1963. It lasted from the end of the preceding December until well into March. In my Surrey garden we did not see our lawn for 10 weeks: it was hidden by several inches of frozen snow. I was not doing 0-group studies then, but scale studies subsequently (on bass caught all round the coast) showed that no bass of the 1962 class, then into their first winter, had survived. They also indicated that spawning in 1963 had been a total failure – due no doubt to the excessively low sea temperatures. Probably the eggs did not develop sufficiently to shed and were resorbed into the fish.

All round the coast there were reports of iced-up estuaries, of heavy casualties among the inshore fish and shellfish. Typical were these reports from the Fishing Gazette –

  • Skegness – Lobsters and large conger washed ashore; icebergs in The Wash.
  • Southend – Pierhead surrounded by ice which extended well down the Thames estuary.
  • Folkestone – Many conger floating 3 miles offshore, the “slick” extending nearly to Dungeness, where one fisherman gaffed 150 stone of still-living specimens.
  • Wittering – Beach ankle-deep in razor fish and limpets.
  • Isle of Wight – Great numbers of school bass, mullet, wrasse &c on beaches.
  • Looe  Estuary – Frozen over from Banjo Pier up.
  • Colwyn Bay – Harbour and estuary frozen solid.

Worse than the actual casualties among young bass were the long-term effects. They did not become apparent until three years later, when my scale &c studies showed that first spawning by bass just coming up to maturity had been retarded for up to three years.

Other – less savage – cold spells followed in the early months of 1965, 1968, 1969, 1970, 1974 and 1979. They resulted in weak classes either of the preceding year (where poor summer had retarded growth) or of the year when the cold spell occurred (if spawning had been poor). The weak classes resulting were 1965, 1968, 1970, 1974 and 1978. There were then no real alarms until 1986, when the quite severe 5-week spell caused the losses already noted.

After that there was a long period of mainly mild winters until quite recently. In December 1995 we had two spells totalling 21 days with temperatures well below normal; then a continuous one in the succeeding (1996/97) winter which lasted 19 days. Neither of these appeared to cause casualties – not even among the 25% of the 0-groups which in the second case were still below 60 mm at the onset of the cold. That gives another pointer to the dividing line between harmless and damaging cold spells. Perhaps 4 weeks is the rough line above which the under-60’s are unlikely to survive.

The broad effect so far as recent history is concerned is that since 1988 there has been a succession of mainly good year-classes, apart from occasional problems with offshore winds at time of entry to the nurseries. This was particularly noticeable with the 1991 class in south side nurseries of the south-west. Prolonged northerly winds in early summer resulted in very weak classes in Tamar, Fowey and Fal – but not in Camel and Taw on the north side.

A major consequence of these mainly mild winters has been that the North Sea is getting warmer. The effect is aided by the presence of power stations on the east coast (discharging their warmed-up cooling water), by the network of warm oil-pipelines strung across the sea-bed, and perhaps also by thermal seeps. For bass the effect on recent superabundant classes has been that they go much further north up the U. K’s eastern coast. They are now reported regularly from our north-eastern and Scottish coasts. It is even possible that some are going right “round the top” and getting to the food-rich areas of north-western England like Morecambe Bay. It could explain the presence there in 1997 summer of bass which showed a faster growth-rate than is usual there: more like the south-east growth rate. But much more research is needed to confirm this (or otherwise).

From ‘Life with Bass’ by Donovan Kelley

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