Investigations of the Juvenile Herring Stocks in the Strait of Georgia Region, 1955
Total Page:16
File Type:pdf, Size:1020Kb
INVESTIGATIONS OF THE JUVENILE HERRING STOCKS IN THE STRAIT OF GEORGIA REGION, 1955 .. by Alan s. Hourston 1956 bio logical S tation Nanaimo , B.C., TABLE OF CONTENTS I Introduction II Population Estimates 2 A. Methods 2 B. Results 3 III Growth 5 A. Methods 5 B. Results 6 l. Introduction 6 2. Age-length relationship 6 3. Age -weight relationship 7 4. Weight-length relationship 7 IV Relationship to Adult Populations 8 A. Introduction 8 B. Tagging during 1955 8 c. Recoveries during the 1955-56 fishing season 9 V Relationship to Environment 10 A. Introduction 10 B. Topography lO c. Temperature and salinity ll D. Plankton ll VI Summary ll VII Acknowledgements 13 VIII References 15 APPENDIX 1 - Average juvenile herring populations on the lower and 16 middle east coast of Vancouver Island forthe period 1940-1949 and in the San Juan Islands-District No. l Region tor 1953 as determined from the abundance of adult herring. Page APPENDIX 2 - Factors affecting the survival of tagged herring. 18 {a) Differences in the condition of the fish prior to tagging 19 (b} Differences in the effect of the tagg~ng operation 19 (1) Changes in general tagging technique 19 {11) Differences between lots of tags in the condition of tags 20 (iii) Differences in the technique of individual taggers 20 (c) Differences in the effect of the environment after tagging 22 APPENDIX 3 -- Extent of mixing indicated by recoveries of adult herring 23 tagged as juveniles during a survey in Barkley Sound (1951-54). (a) Introduction 23 (b) Efficiency of reduction plant magnets in recovering 24 juvenile herring during the 1955-56 fishing season (c) Recoveries of tags during the 1955-56 fishing season 25 {i ) Introduction 25 (ii} Homing by area 26 (iii) Homing by sub-district 26 APPENDIX 4 - Relationship of juvenile abundance to that of plankton 27 organisms in Barkley Sound during 1954 •. I INTRODUCTION An investigation the population dynamics of juvenile herring in the Gulf' of Georgia was initiated in 1955 with a view to predicting recruitment to the adult populations in that region. A preliminary study carried out in Barkley Sound on the west coast of Vancouver Island indicated that relative abundance or juveniles can be assessed by a survey method {Hourston, 1956). However, tags inserted in juveniles and recovered from adults taken in the fishery indicated a relatively great scattering of the herring be- tween the time of their offshore migration in the fall of their first year to their recruitment to the adult stocks; only about half of the tagged fish "homed". Consequently the unit of popula- tion of such a study within the Gulf of' Georgia should probably not only include the entire Gulf, but also neighbouring regions, such as Discovery Passage and Barkley Sound. The 1955 program was mainly exploratory. Facilities for surveying the entire Gulf' were not available so stress was laid on locating as many of the major stocks as possible. Since the adult population in the lower east coast comprises the majority of' the adult herring in this region, a relatively complete survey was made of that sub-district and adjacent waters (San Juan Islands, American mainland and District No. 1). The survey of the middle east coast sub-district was incomplete, but inclµded the most promising localities. No survey was made of' Barkley Sound as the general picture for that area had been' determined for the previous four years {Hourston, 1956). - 2 - A tagging program was initiated in the area surveyed to determine the relationship of its juvenile stocks to the adult populations. Observations of temperature and salinity were made during the survey in an effort to relate juvenile abundance to environmental conditions. Gertain additional data on the preliminary Barkley Sound study which were not available when this study was reported (Hourston, 1956) have been appended to this report. These include returns of tags inserted in juveniles in Barkley Sound and an analysis of the abundance of potential juvenile food organisms. II POPULATION ESTIMATES A. Methods The juvenile herring population in each locality surveyed was estimated by counting the number of "average" schools observed flipping at the surface and showing on the echo-sounder tracings. Juvenile herring schools may be readily observed flipping at the surface for about a two-hour period at dawn and at dusk. Scouting was confined to this period. A survey conducted in Barkley Sound (Hourston, 1956) indicated that after mid-July the juveniles tend to form schools of about 10,000 fish. Such an "average" school showed several flips at a time, generally in a circle about one to two feet in diameter, and occupied a surface area of' about 20 to 30 feet square. In late summer and early fall the schools often mingled to form large masses of fish; sometimes over a million fish would be schooled together in this manner. These bodies of fish were estimated in terms of the number of "average" - 3 - schools they contained from the number of circles or "groups" of flips showing, and from the area they occupiedo When weather conditions were unfavourable the juveniles tended to stay farther below the surface. Also when the fish were very abundant, they usually extended to considerable depths. Schools below the depth of the draught of the scouting vessel (eight feet) were estimated from the marks they made on echo-sounder tracings. The Barkley Sound survey (Hourston, 1956) indicated that an "average" school (i.e., 10,000 fish) makes a mark about 1.5 millimeters deep (the distance between two fine lines on Bendix sounder paper) and one to three scratches wide when passed over at a speed of seven to nine knots. Since other species present at this time of year tend to be found in different types of localities and under different conditions, and since their behaviour, and hence the type of mark they produce, is somewhat different. to that of the young herring, their confusion with juvenile herring was unlikely. Also juvenile herring form by far the largest part of the young fish stocks in these localities at this time of the year, so any errors.resulting from mistakes in identification should be small. B. Results Estimates of the juvenile herring stocks were tabulated by region; namely - San Juan Islands and adjacent mainland waters (Table I}, District Noo l {Table II), .the lower east coast (Areas 17A 17B 18 and 19} of Vancouver Island {Table III), and the middle east coast (Areas13, 14, 15 and 16) of Vancouver -4 - Island (Table IV)o Localities scouted are shown in Figure 1 and juvenile rearing grounds in Figure 2. The middle east coast survey was incomplete. The totals for each region are tabulated below, along with average juvenile abundance in the lower and middle east coast sub-districts for the ten year period 1942-43 to 1951-52 calculated from the potential number of recruits at Age III and natural mortality rates between Age I and Age III (Appendix I). Comparable data were not available for the San Juan Islands and District No. 1, but.a rough estimate based on spawn deposition in 1955 was included. Region Abundance ( x'l06) Observed, 1955 Calculated AverageMaximumMinimum San Juan Islands 60.09 1042 .. •• District Noo 1 21.93 Lower east coast 168.54 725 1014 437 Middle east coast 38032 346 731 115 The estimate for the San Juan Islands-District No. 1 region seems unduly low. Since data from this region are sparse, the estimate was considered unreliable. The middle east coast estimate was also considered unreliable as the survey was in- complete. However, data for the lower east coast should be compar- able. Information gathered from interviews with local residents and fishermen indicate that abundance of juveniles in 1955 was much higher than in the immediately preceding years. Thus survey esti- mates probably covered about one-fifth of the fish present . This - 5 - compares well with estimates on completeness of scouting surveys for Barkley Sound in 19.51 1/5.5 and 19.52 (1/7.5} based on simultaneous Schnabel-type marking experiments (Hourston, 1956 butnot forthe 1953 and 1954 surveys• The 1953 estimates were considered low but the 1954 estimates were consideredrepresenta- ti ve ( 1/25). However calculations. similar to the above gave observed and calculated abundances of 24.98 and 31.8 million fish respectively, indicating a 1.15 ratio to be more likelyo This higher ratio may be related to the tendency of the Barkley Sound fish to be scattered in smallergroups However, it is the relative rather than absolute estimates of year-class strength which are of primary interest. Surveys of Barkley Sound indicated that the 1952 year-class was below average strength as juveniles and the 1953 year-class was above average. At recruitment the 1952 year-class was well below average strength and the 1953 year-classwell above average. According to local reports the 19 55 year-class in the Strait ofGeorgia is above average in strength but nno direct comparison can be made until thecompletion of another survey in 1956 . III GROWTH A. Methods Small samples (1-30 fish) of juvenile herring were taken by herring rake occasionally during the scouting survey. Samples of over 100 fish were taken from each school seined during the tagging program. Other investigators contributed samples taken by beach seine and by otter trawl in the San Juan Islands. All - 6 - samples were preserved in 5 percent formalin. Each sample was dumped onto a tray, thoroughly mixed, and, working from one side across the tray, 100 fish were removed and their standard length measured to the nearest 0.5 mm.