Epsilon Archive for Student Projects

Abstract

Anadromous burbots Lota lota (Linnaeus 1758) from two similar rivers in the Gulf of Bothnia area, Sävarån and Rickleån, Northern Sweden, were compared. The aim was to test for and explain differences in ecology. Spawning burbots in Sävarån were smaller, younger, grew slower and were more densely populated. The positive correlations between individual size, and gonad- and liver indexes which were found in Rickleån, were weaker or absent in Sävarån. The positive correlations of rapid growth in regard of liver tissue growth and K, were weaker in Sävarån than in Rickleån. K in Sävarån were not correlated with individual size, and were also lower in general, than in Rickleån. The majority of spawning burbots had empty stomachs, 73 % in Sävarån, and 58 % in Rickleån. “Rest year” strategies, which were correlated to both organ (except gonads) indexes, and K, where common in Rickleån, but almost absent in Sävarån. Almost all burbots, even those smallest, found in the stomachs of bigger individuals, where fully mature with high gonad index. The size group histogram and normal probability analyses, indicated that there were large numbers of even smaller individuals present at the spawning sites in Sävarån, not possible to sample with the gear used. As a whole, burbots in Sävarån exhibited precocity, favouring early spawning on the expense of growth and/or higher fecundity in future spawnings. I also found that many of the differences confirmed between the populations, were also present within both populations, between males and females. Males exhibited a more precocious reproduction, while females exhibited a more delayed reproduction. [1] The differences between the populations in age at spawning, individual size, growth,
allocations to different organs and foraging, were presumably due to both
a) trade-offs favoured by selection due to differences in mortality related to human harvest, and
b) density-dependent effects [2].
[2] The population densities differed, assumably due to two abiotic factors,
a) higher summer temperatures in the littorals outside Sävarån and
b) more significant episodical acidity in the spring in Rickleån.
[3] The demise of the largest and oldest burbots in these rivers, as well as the whole Sweden, was a different problem. The climate change seemed to be the most plausible explanation. A climatologic and thermal threshold for burbots may have been exceeded.
Whenever a new discovery is reported to the scientific world, they say first, “It is probably not true.” Thereafter, when the truth of the new proposition has been demonstrated beyond question, they say, “Yes, it may be true, but it is not important.” Finally, when sufficient time has elapsed to fully evidence its importance, they say, “Yes, surely it is important, but it is no longer new.”
Michel de Montaigne (1533-1592).