• Studies of the Eel Anguila Anguila in Ireland No. 4. In the Munster Blackwater River

      Moriarty, C. (Department of Agriculture and Fisheries [Fisheries Division], 1974)
      Yellow eels from estuarine and freshwater populations were sampled by fyke netting: in 1965 and 1966 2,221 specimens from the estuary and in 1972 and 1973 826 specimens from freshwater. It was the first extensive study of eels in an Irish river and the population was found to be more dense than that recorded in lakes. Growth in the estuary was relatively fast and spawning migration began at 9 years while in the freshwater growth was slower and migration began about four years later. One specimen of 36 years old was found and more than 17% of the freshwater sample were over 19 years. Eels of less than 40 cm fed largely on invertebrates whereas eels of 50 cm and over fed mainly on fish, cyprinids being taken to a much greater degree than salmonids. It was calculated that fyke netting of the unexploited fishery could yield a catch of 21 tonne of eels of over 50 em length in the lowland freshwater portion of the river.
    • Studies on the Minnow Phoxinus Phoxinus (L.) from an Upland Irish Reservoir System

      Dauod, H. A.; Bolger, T.; Bracken, J. J. (Department of Agriculture and Fisheries [Fisheries Division], 1985)
      The paper deals with the population structure and biology of the minnow, Phoxinus phoxinus (L.), in the Roundwood Reservoir system. A total of 4,342 minnow were taken during the study period from three locations, 2,796 fish were used to calculate the age distributions and 3,013 were examined for gut contents. The age data, determined from the otoliths, showed that there are five age classes present. The mean length at the end of the first year (O-Group fish), was 3.41 cm, at the end of the second year (I-Group fish) it was 5.32 cm and 6.68 cm at the end of the third year (II-Group). Only thirteen minnow were found to be older than three years. The breeding season is later than normal and peaks in August and September. Sexual maturity is reached by the majority in their second year and all older fish are mature. Egg diameters were shown to be smaller than for fish from midland Irish waters. In the North and South Lakes the dominant food organisms were found to be chironomid and trichopteran larvae, molluscs and Cladocera. The diet of the fish from the Vartry River contained more ephemeropteran nymphs. Seasonal differences in the diet are noted. The influence of the minnow on the associated fish species is discussed. An account of the distribution of the minnow in Ireland is included as an Appendix.
    • Studies on the Three-Spined Stickleback Gasterosteus aculeatus L. from an Upland Irish Reservoir System

      Dauod, H. A.; Bolger, T.; Bracken, J. J. (Department of Agriculture and Fisheries [Fisheries Division], 1985)
      Monthly samples were taken using a small-meshed beach seine. A total of 1092 sticklebacks were captured, 725 fish were used to calculate the age distributions and 699 were examined for gut contents. The age data, determined from the otoliths, showed that there were four age classes present. The mean length of first year fish (O-Group fish) was 2.24cm, of second year fish (I-Group) 3.36 and 4.47cm and 5.72cm for II-Group and III-Group fish respectively. The breeding season was June-July. Sex ratios were similar in both lakes and did not differ significantly from 1 : 1. Sexual maturity was reached by all fish above 3cm. The smallest maturing virgin was 2.6cm in length. Egg diameters varied between 1.0 and 1.5mm. The diet was similar in the two lakes studied. In the North Lake Cladocera, chironomid larvae, copepods and molluscs dominated while in the South Lake Cladocera, copepods, chironomid larvae and surface insects were dominant. Sticklebacks were extremely scarce in the Vartry River and feeder streams entering both lakes and would not pose a threat to egg production by trout and minnow in these streams. The influence of sticklebacks is discussed in relation to other fish species.
    • Temperature and Oxygen Determinations in some Irish Lakes

      Fitzmaurice, P. (Department of Agriculture and Fisheries [Fisheries Division], 1971)
      Information about temperature and dissolved oxygen profiles in Irish lakes has hitherto been lacking. During the period 1966 to 1969 data on temperature and dissolved oxygen content at various depths were determined for a number of Irish lakes. The results are summarised in this paper.
    • Water Quality Investigations in the River Blackwater and River Martin, Co. Cork--1966-1969

      Toner, P. F.; O'Connell, C. (Department of Agriculture and Fisheries [Fisheries Division], 1971)
      Investigations in the period 1966 to 1969 showed that the bulk of the waste discharged to the Blackwater and Martin is of an organic nature and arises mainly from industries processing milk, sugar-beet and other foods and to a smaller extent from domestic sewage. Pollution, indicated by increases in the biochemical oxygen demand and suspended solids and depletion in dissolved oxygen, was detected below the towns of Rathmore and Mallow on the main Blackwater, Mitchelstown, on a tributary of the Blackwater, the R. Funcheon, and Rathduff on the R. Martin. Depletion of dissolved oxygen sufficiently large to constitute lethal conditions for fish and other aquatic life was recorded only at Mitchelstown and it appears that the high rates of reaeration operating in most cases prevented more widespread deoxygenation and also reduced the extent of diurnal variation. The concentrations of suspended solids recorded in polluted reaches were well below those which are directly injurious to fish but the accumulation of this material as sludge in slow flowing stretches below outfalls constitutes an extra demand on dissolved oxygen which in combination with the B.O.D. of the water may lead to lethal conditions. In the Funcheon, the presence of this material and of sewage fungus on the substratum invalidates the application of the standard theoretical method for predicting the variation of dissolved oxygen. The partial or complete elimination of the normal flora and invertebrate fauna from riffles below the main waste outfalls, and replacement of these by biocoenoses typical of slow flowing silted reaches were recorded in each area. The extent of such changes seemed to be related more to the intensity of sewage fungus growth on the substratum than directly to the chemical quality of the water. Complete elimination of fish was only recorded at Mitchelstown, the stretch affected being half a mile in length. Trout and coarse fish were present in all af the other polluted stretches investigated though in some of these young salmon were absent and trout very few in number. Trout appeared to make better growth in polluted than in unpolluted water, especially at Rathmore, and this is due in part to a greater food supply in the former reaches. Short surveys of the Blackwater estuary indicated that this reach may be slightly polluted. While the investigations indicated that severe pollution occurred in several reaches of the Blackwater and Martin, it was considered that the overall productivity of fish in the rivers was unlikely, at that stage, to have been adversely affected by such pollution.