THE AGE, GROWTH AND MORTALITY OF THE, •LEMON SOLE (Paropnrys vetulua Girard) ON THE (BRITISH COLUMBIA. FISHING GROUNDS BY Keith S. Ketchen A Thesis submitted in Partial Fulfilment of (The Requirements for the Degree of MASTER OF ARTS in the Department of ZOOLOGY THE UNIVERSITY OF BRITISH COLUMBIA April, 1947. ABSTRACT Part of the general investigation "being conducted by the Fisheries Research Board of Canada into the condition of the Pacific coast otter trawl fishery deals with the length and age analysis of the species of fish caught. The age of one of these species, the lemon sole, Parophrys vetulus GIrard has been determined through a study of the otoliths or ear- stones. In general the older fisheries and those closest to large Canadian and American markets produce the youngest and the smallest fish. Soles of four and five years of age pre• dominate in the catches from the Strait of Georgia and from the west coast of Vancouver Island, The comparatively recent fisheries of Queen Charlotte Sound and Hecate Strait show a predominance of six and seven year old fish. The male lemon sole reaches an age of ten years and a length of 400mm., while the female reaches twelve or thirteen years of age and grows to a length of over 500mm. The female sole, between four and eight years of age grows approximately 11 mm. more per year than the male. The fish in the Strait of Georgia and in northern Hecate Strait have a slightly greater growth rate than those on the west coast of Vancouver Island and in Queen Charlotte Sound. Marked differences in total annual mortality rates have been shown. In the Strait of Georgia and on the west coast of Vancouver Island the rate is between (>0% and 70f. In Queen Charlotte Sound the rate is 52%, and in northern Hecate Strait it is between 30% and 40f». The relative recency of the northern Hecate Strait fishery has raised the suggestion that the total mortality rates of 30?« in female fish and 39% in male fish approach the natural mortality rate. Dominance of the 1939 year class has been observed in the s study of fish taken during ,1944, 1945 and 1946 in northern Hecate Strait. In the last year another strong year class, that of 1942, made its appearance. The relative lack of success in the brood years of 1940 and 1941 was the factor responsible for the absence of small fish in the catches during the fishing seasons of 1944 and 1945. TABLE OF CONTENTS . .Page INTRODUCTION 1 MATERIALS AND METHODS . 7 LIFE HISTORY 13 THE ANALYSIS OF THE COMMERCIAL CATCHES OF LEMON SOLES 18 Strait of Georgia 18 West coast of Vancouver Island.... 23 Q,ueen Charlotte Sound 26 Hecate Strait...................... 27 GROWTH RATES ........................................ 34 MORTALITY RATES 39 DISCUSSION 44 Growth 44 Mortality. 47 The relationships of mortality and fluctuations in strengths of .year .classes to .age ~' composition. •• 52 SUMMARY AND CONCLUSIONS 56 ACKNOWLEDGMENTS 60 REFERENCES 61 APPENDIX 63 - 1 - I. INTRODUCTION On the Pacific coast of Canada during the past six or seven years there has "been witnessed an extensive development of the trawl fishery. This expansion has arisen mainly as a result of the need for fish products, both here and abroad, to alleviate the shortages in meat supplies brought about by war• time conditions. The Fisheries Research Board of Canada, fore• seeing the possibility of over-exploitation of this new in• dustry and of its possible conflict with other marine fishing methods, in 1943 began an investigation designed to accumulate information which could be employed in a sound program of con• servation and management. One of the first objectives in studying the condition of the trawl fishery has been to determine the nature and abun• dance of the species of fish captured, and to determine the factors which limit this abundance. The Problem One of the principal species of flatfish landed by ottarr trawlers is the lemon sole (Parophrys vetulus Girard), a fish of considerable market importance because of its abundance and the high quality of its flesh. As part of the general invest• igation being conducted on this species, primary consideration is given in this work to the study of the size and age compos• ition of catches landed at important British Columbia ports during 1945 and 1946. Analysis of this sort is an essential requirement in revealing the condition of the fishery in that it shows in what way production is influenced 1. by the general age distribution in the various populations, 2. by the growth rate, 3. by the mortality rate, and 4. by fluctuations in the strength of year classes brought about by varying successes of brood years. These aspects will be considered in detail, along with important related subjects on size and age at maturity, reproductive ca• pacities, and the growth and habits of young, post-larval fish. Afie Determination, The age of the lemon sole has been determined through ex• amination of the otolith, a small hard, calcareous body situated in the saccular portion of the ear. The method has long been employed by European investigators of the North and Baltic Sea fisheries. Such men as Graham (1928) and Van Oosten (1941) however have emphasised that too often the reliability of age determination in one species of fish is used as justification of the reliability in another, a practice which could undoubtedly lead to substantial error in results. With this in mind the study of the lemon sole otolith has been approached with con• siderable caution. In a preliminary investigation of this sub• ject by the writer (1945 a) measurements of the otolith showed a fairly constant ratio to the length of the fish. Further studies of the alternating light and dark rings appearing on the otolith have been found to form reasonably consistent pat• terns related to annual temperature changes. On the basis of these correlations, the otolith of the lemon sole is believed to give a fairly accurate picture of age, especially for the lower and middle age groups. - 3 - History Qt Afte pe^rgina^gn, Graham (1928) in his review of the literature states that Reibisch investigating the North Sea plaice in 1899 was one of the first to suggest that the differential structure of the otolith Is related to temperature. However Cunningham (1905) was the first to show that the opaque or white ring represents the summer growth and that the transparent or dark ring repre• sents that of the winter. Wallace (1905) also investigating the plaice fishery gave statistical evidence supporting the findings of Cunningham. In 1916 Storrow extended age determina• tion by this means to other species of flatfish. Since that time the method has spread to the study of other types of ground fish, the hake by Hickling (1933)» and cod and haddock by many- workers seeking correlations with scale readings. On the Pacific coast of North America most noteworthy advances in this field have been made by the International Fisheries Commission with respect to the study of the age of the halibut. Apparently the only published work on flatfish other than halibut from this coast has been by Smith (1936) in which he gives a general sur• vey of the ages of several species taken in Puget Sound waters. HlSfrffY off *fre ?ra.wl Pigftgyy The trawl fishery for flatfish and lingcod on the Pacific coast of Canada began about 1912 and was confined mainly to English Bay and the waters adjacent to Point Atkinson and Point Grey, regions all within sight of Vancouver. According to the older fishermen the fish were very plentiful in those days, but because of a very limited market the occupation was pursued by only one or two boats. However, during the first World War an increased demand saw the fishing grounds extended around Point Grey and south along the Sand Heads of the Fraser River estu• ary. In the same period three old country-type steam trawlers began operating out of Prince Rupert in northern Hecate Strait. By 1921 however these boats had been converted to other employ• ment as a result of a collapse of the market to its pre-war level. On the southern part of the coast fishing was continued by a few boats from Vancouver, and by 1923 the grounds had been extended from the Fraser estuary to the waters among the adja• cent islands in the lower portion of the Strait of Georgia. The Baynes Sound trawling grounds, along the east coast of Vancouver Island and about forty miles north of the Gulf Islands were not discovered until 1930 and did not receive much attention until 1938 or 1939. In 1937, with the improvement of boat designs and engines, one or two boats began operating in the summer season off the southern part of the west coast of Vancouver Island near Port San Juan. By 1939 the number of boats had increased to eleven and the grounds were extended northward to Barkley Sound. With the discovery of the high vitamin A content in dog• fish liver oil, large American boats which had previously been centered on the Swiftsure Banks off Cape Flattery moved north• ward ln 1940 to the La Perouse Bank off the west coast of Van• couver Island. By 1942 these boats far outnumbered the smaller Canadian boats, and within a few years had made their appearance on the Queen Charlotte Sound and Hecate Strait grounds. The Goose Island grounds in Queen Charlotte Sound were opened up in the early years of the second World War, but Fig. 1 Parophrys vetulus Girard because of the long distance from port and the severe weather conditions prevailing year-round in these waters, only large trawlers have been able to fish with any degree of success.