Pitfall Trapping of Microtus townsendii
A field population of Microtus townsendii was trapped concurrently with live-traps and pitfall traps throughout the spring and summer of 1975. The population was at peak densities, with a maximum of 519 voles per acre. The number enumerated by pitfalls was up to two times larger than the number enumerated by live-traps. Capture of a large number of young in pitfalls indicated that mortality was higher among post-weanlings than among any other age group. Recruitment to live-traps was largely restricted to animals over 40 g, whereas in pitfalls animals were generally lighter. Over half of the 1,100 animals captured first by pitfalls were never caught by live-traps. Among those subsequently entering live-traps, many stayed away for 6 to 10 weeks, and some of the smaller ones stayed away even longer. Few animals dispersed to other nearby trapping grids. Pitfall trapping and live-trapping sample different subpopulations, young animals and transient adults being a large fraction of the individuals caught in pitfalls.
A pulsed-removal experiment on the vole Microtus townsendii
A grassland population of Microtus townsendii was subjected to an alternating sequence of 1 month total removal and 2 months mark and release for 2 years. The demographic performance and aggressive behaviour of voles from this pulsed-removal area were compared with those of voles from an adjacent control population. The population on the pulsed-removal area increased rapidly from immigration at all times of the year, regardless of whether the control population was increasing or declining. Voles that colonized the pulsed-removal area were able to breed and survive as well as, or better than, voles from the control population. Males and females from the pulsed-removal area showed more submissive behaviour when fought in a neutral arena against resident control animals. The subordinate behaviour pattern of pulsed-removal colonists was detectable in voles tested 6–8 weeks after colonization. Thus we have satisfied two necessary conditions to show that spacing behaviour limits the breeding population of this ...
The incidence of dead and dying fish in the Moray Firth, September 1963
Dead and dying fish were reported from the Moray Firth during a period of 15 days in September 1963. The initial mortalities were in the inner Firth, but they later spread outwards, especially eastwards along the southern shores of the Firth. Mass mortalities of the lugworm (Arenicola marina) also occurred in Scottish waters in 1962 and 1963, and at Balintorein Easter Ross fish and lugworm mortalities were contemporary.