Dear John (on behalf of T.D.A.S)
I am writing with a quick summary of the data from the loggers that were put in T.D.A.S lake earlier this year. The loggers were positioned on the far side of the island and under the willow tree at the dam end. Temperature and Dissolved Oxygen (DO) were measured every fifteen minutes for two months. I haven’t enclosed the raw data as each month’s results for each logger are 144 pages long! However, I can email the data if required.
The purpose of the exercise was to see whether it was temperature that was limiting the survival of carp fry within the lake. Literature suggests that carp eggs need prolonged (2 weeks +) water temperatures above 20??C to survive and successfully mature as fry. Also it is generally suggested that carp spawn when the water temperature reaches about 16?C.
March - May
Water temperatures were fairly constant at 10-15?C from late March (when the
loggers were put in) to the third week in May.
DO levels during this period were good and showed a regular daily fluctuation between roughly 55% DO at night and 85% DO in the day. This is the effect of the aquatic vegetation in the lake (including lilies, rushes, sedges, iris, algae etc) photosynthesising (oxygenating the water during the day and re-absorbing it at night).
May - June
Towards the end of May air temperatures increased quickly and this was mirrored
in a steady increase from roughly 14?C all the way up to 20?C for a few hours
on 02/06/09. Roy Jones saw carp beginning to spawn during this period and this
concurs with the 16?C ‘rule’ for carp spawning. However, during
this period of warmer weather the night-time water temperatures in the lake
were still low (around 15?C). The air temperature during the next three weeks
also fell and these two factors combined mean that any eggs fertilised during
this period are likely to have been moribund.
The warmer water temperatures during this period had a clear effect on DO levels with the daily fluctuation changing to around 30-40% at night up to 110% during the day. These levels of DO are reasonable and would appear to show the added effect of an algal bloom brought on by the warmer weather.
June - July
Towards the end of June and into July the water temperature really warmed up
in the period of very hot weather and, for a few weeks, the water temperature
did not get lower than 19?C. I saw carp spawning at this time and, given the
conditions in the lake, these eggs/fry should have survived.
The DO during this period was more variable but within the range 30-100%.
Summary
The data from the loggers shows that the water temperature in the Dell would,
in an average summer, easily get high enough to induce carp to spawn but it
takes a period of sustained, very hot weather (like we saw in July) to maintain
the lake at an optimum temperature for carp fry survival. This has occurred
this year but is unlikely to happen every summer due to our changeable climate.
The measurement of DO was an added bonus of the loggers and shows that the daily fluctuations in DO levels are fairly constant and, although the range (eg 30-100% in a day) is a large one, there seems to be no negative impact on the fish population. However, there were a couple of days during the study where the DO unexpectedly dropped very steeply to levels that could cause problems. The worst of these was on 29/05/09 where the DO briefly dropped as low as 18.3% at 6:15am. Again there seems to have been no negative impact on the fish population, perhaps due to the short-lived nature of the DO slump (levels were only under 30% for 3 hours). I would recommend keeping a close eye on the lake over the spring/summer (especially when air temperature rises quickly as it did at the end of May and the end of June) and if fish show signs of distress (this is likely to happen very early in the morning when DO levels are at their lowest) then use your aeration equipment.
With the work that has already been undertaken (spawning brushes, marginal vegetation, floating island, tyre reefs etc) to improve the survival of juvenile fish in the lake, any carp eggs that do hatch have a much greater chance of reaching maturity. If the carp spawning this year has been successful then hopefully a few fingerlings will show in catches next year.
In terms of DO it appears to be the influence of algal blooms brought on by warmer weather that has caused the very low DO levels. This can be seen in the regular diurnal fluctuation in DO levels from 55-85% from March to May. This would have been caused by the photosynthesis of the marginal aquatic plants and the lilies with little influence from algae. The warmer weather at the end of May brought on an algal bloom and the added DO removed and then re-absorbed into the water caused DO levels to fluctuate more (30-110%). It is this ‘extra’ fluctuation in DO levels caused by algae that can be damaging to a fishery.
The most sustainable ways to control algae are to use barley straw and maximise the growth of aquatic plants (both of which T.D.A.S already do). While providing benefits in terms of aesthetics and cover for fish, aquatic plants (lilies, reeds, sedges, iris etc) perform an important function by absorbing nutrients from the lake to the detriment of algae.
Yours sincerely
Dave Bartlett
Fisheries Technical Officer