ess Fis

, oc ess J.F. Caddy Invertebrates & arine Plants Resource Branch Halifax, .8., B3J 287

IVlslon

February, 1979

Fis

a

•

48

Fi berie and arine Service anuscript Reports The e report contain cientific and te hni al information that repre ent an important contribution to e i ting kno ledge but hich for orne reason rna not be appropriate for primar cientific i.e. Journal) publication. The differ from Technical Report in term of ubject cope and potential audience: anu cript Report deal primaril ith national or regional problem and di tribution i generall re tricted to in titution or indi idual located in particular region of Canada. 0 re triction i placed on ubject matter and the eries reflects the broad intere t and policie of the Fisheries and arine Ser ice namel fi herie management technolog and de elopment ocean cience and aquatic en ironment rele ant to Canada. anuscript Report rna be cited a full publication. The correct citation appear abo e the ab tract of each report. Each report will be ab tracted b Aquatic Sciences and Fisheries Abstracts and ill be inde ed annuall in the er ice inde to cientific and technical publication. umber 1-900 in thi eri ere i ued a anus ript Report Biological eries of the Biological Board of Canada and ub equent to 1937 hen the name of the Board as changed b t of Parliament a anuscript Report Biological erie) of the Fi herie R earch Board of Canada. umber 901-1 25 ere i ued a anu cript Report of the Fi heri Re earch Board of Canada. The eri name a hanged ith report number 1 26. Detail on the a ailabilit of anuscript Report in hard cop rna be obtained from the i uing e tabli hment indicated on the front 0 er.

Service des peches et de la mer Rapport manuscrits Ces rapport contiennent des renseignement cientifique et techniques qui con tituent une contribution importante aux connai ances actuelles mai qui pour une rai on ou pour une autre ne emblent pa appropri' pour la publication dans un journal cientifique. II e di tinguent des Rapport techniqu par la portee du ujet et Ie lecteur i e' en effet il attachent principalement a de problemes d ordre national ou regional et la di tribution en est generalement limitee aux organi m et au per onne de region particulieres du Canada. II n y a aucune re triction Quant au ujet· de fait la erie reflete la aste gamme d interet et d politique du Servi e de peches et de la mer notamment gestion de pe he . techniques et de eloppement cienc oceanique et en ironnement aquatiques au Canada. Les anuscrit peuvent etre consideres comme de publication completes. Le titre e act paraJ't au haut du resume de chaque rapport qui era publie dans la revue Aquatic Sciences and Fisheries Abstracts et qui figuera dan I index annuel des publications cientifique et techniques du Ser ice. Les numero de 1 a900 de cette erie ont ete publie a titre de manuscrit (Serie biologique) de I Office de biologie du Canada et apres Ie changement de la designation de cet organi me par decret du Parlement en 1937 ont ete clas e en tant que manuscrit (serie biologique) de I Office de recherche ur les pecheries du Canada. Les numero allant de 901 a1425 ont ete publies a titre de manuscrit de I Office des recherche ur les pecheries du Canada. Le nom de la erie a ete change a partir du rapport numero 1426. La page couverture porte Ie nom de I etablis ement auteur ou I on peut e pro urer les rapport ous couverture cartonnee. • Cover design by Chns me Rusk

Department of Fisheries and Oceans Manus Report No. 1489

February, 1979

SOME CONSIDERATIONS UNDERLYING DEFINITIONS OF CATCHABILITY AND FISHING EFFORT IN SHELLFISH FISHERIES, AND THEIR RELEVANCE FOR STOCK ASSESSMENT PURPOSES

BY .} .F. Caddy

Resource Branch Dept. of Fisheries & Oceans P.O. Box 550 Halifax, N.S. B3J 2S7

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C9 Minister of Supply and Services Canada 1979 Cat. No. Fs 97-4/1489 ISSN 0701-7618

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TABLE OF CONTENTS

iv

ABSTRACT/RESUME. INTRODUC'f'TON .

Page

.

Definitions of Catchabil

Effort, and Fishing Power

HAND GATHERING Catchability and Gear Selectivity. Effort Definition . DREDGES AND MECHANICAL HARVESTERS. Selectivity and Catchability Coefficient . Factors Affecting Dredge Selectivity and Efficiency . . Definition of Effort in Dredge Fisheries . . . . . . TRAWL FISHERIES. Fishing Power. Factors Affecting Catchability and Gear Selection. Corrections to the Effort Unit ......•. 'rRAP FISHERIES . Fishing Power. Nominal Units of Effort. Corrected Effort Units . . Corrections for Environmental Factors and Behavior Corrections for Catchability and Gear Selection. . Physiological and Behavioral Considerations . . . .

4 4 4 6 6 6 7

8

8 8 9

9

10 10 10 11 12 12

SUMMARY . .

12

REFERENCES

13

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ABSTRACT ,T.F. 1979. Some considerations and fi effort in shellfish fisheries assessment purposes. Fish. Mar. Servo MS

definitions of catchabi1ity for stock and their 1489.

The lems of and effort definition in invertebrate fisheries have been reviewed from available literature on four broad of fisheries hand and fisheries. units of nominal effort have been proposed power and coefficient identi These include a of , environmental, and behavioral considerations for the species concerned, in addition to the more obvious mechanical considerations which determine the area of influence and efficiency of the gear. Spatial distribution of fishing effort (fishing crr~,r,,,,rr,,) in relation to the distribution pattern of "the determine the effectiveness of a given unit of effort, ane: of gear saturation (especial in and may introduce a density-dependent bias into the definition of fishing effort. Definitions of effort (eg., "days on ground"), which inadequately the process into its of "search time" and "handl time", may estimate the true pressure on the stock, over range of population densities. These sorts of bias misleading when effort data is used in models predicting sustained Whenever it is possible to quantify factors affecting fishing power , it is that they be used to obtain an estimate is additive units, and and rate.

words

catchabi power.

coefficient, s"tock assessment,

RESUME , J.F. 1979. Some considerations and effort in shellfish fisheries assessment purposes. Fish. Mar. Servo MS

definitions of and their relevance for stock 1489.

On a les problemes de definition du de et de l'effort de des invertebres dans la documentation sur quatre grand de peche. Les unites d'effort nominal ont ete proposes, et les facteurs influen~ant Ie pouvoir de peche et Ie coefficient de capture ont provisoirement identi Ceux-ci incluent un de jJuYb..L\.j'..LI...,''::Jiques, environnementales et ethologiques s 'appliconcernees, en des plus la de de La distribution (

-

v -

de la peche) en regard du type de distribution de l'espece peut determiner la validite de I 'unite d'effort choisi, et avec les problemes de saturation d'engin de peche (particulierement pour les et les dragues), peut introduire une erreur influencee par la dens dans l'estimation de l'effort de peche. Les definitions de l'effort (par "jours sur Ie terrain"), qui negligent de subdiviser Ie processus de de recherche" et de manoeuvre", peuvent fausser l'estimation de Ia vraie pression de peche sur Ie stock, en fonction des diverses de Ces genres d'erreur sont particulierement trompeurs quand les donnees d'effort sont employees en modeles predisant Ie rendement soutenu. Chaque fois qu'il est possible de les facteurs influe9ant Ie pouvoir de peche et Ie potentiel de capture, il est suggere qu'ils soient utilises pour corriger I'estimation de l'effort de peche, lequel peut alors s'additionner d'une un de peche a l'autre, et devient proportionnel a l'in-tensite de peche ainsi qu'au taux de mortalite de peche.

- vi -

-

1 -

INTRODUCTION meet assessments of shellfish stocks held in h the need in 1976, the" 64th annual reun the of for further research initiatives to At that shellfish stocks and the definition of relevant effort and it became evident that studies on standardization the gear have gear in relation to fishing exerted marine finnot kept pace with similar studies on gear used for fish, the growing of effort limitation as a method of management in shellfish fisheries , 19 A

1be Shellfish and Benthos Committee the considered the findings of the special meeting on of shellfish s"tocks, and adopted the following resolutions: to the definition of fishing C.Res.19 :5 Attention should be effort for gears cular to shellfish fish0ries and that standard measures should be adopted. C.Res.1976/5:6 The effects of fi shellfish should be given attention.

on the habitat of

This paper attempts a preliminary description and definition of factors relevant to gear performance and fishing effort, while noting that the wide of gears used for shellfish harves a series of definitions each appropriate to a of gear or harvesting

in invertebrate Published data effort and fisheries is not available in a since, unlike the was on these situation with to finfish where ect areas in the 1950 sand 60 s (eg., joint meetings 1957, 1963 ICES effort and the of examination on the measurement of of these areas has been fisheries. The on development in these fisheries to dute has been 1 of new gear des suited to particular fisheries and local conditions. part in the difficulty The lack of standardization plays a of on gear performance.

Definitions of Catchab , two gear characteristics

Effort, and

Power

developed to the of and Dickie, 1964; Gulland, 1964b):

1) What may be termed the description of the mechanics of gear fish behavior in relation to gear. coefficient is defined as q = calA, which individuals in stock area A removed by the gear with efficiency~. This lends itself physical characteristics of the gear (Treschev, gear also on fish behavior relation to stock distribution patterns (which

for first studies of , a catchability of unit area a to measurement of but since effective

- 2 -

this also means that effort units must be an exact fraction of Fto satisfy in the definition the equality F = q'f. This poses major of fishing effort, or more exactly in this case, 2) The approach usually adopted in to choose a convenient, easily measurable unit of nominal (g) (from log records, port interviews), and ust this for power of individual fleet units to arrive at a corrected effort unit (f). The performance characteristics of the gear or units can then be described in terms of the slope of the regression (q) between corrected effort and resultant fishing mortality rate (F) (determined independently from catch curves, cohort is, or tagging ). A full understanding of the factors underlying performance of fishing units comparison between these two approaches. Evidently, however, the definition of fishing effort unit chosen should ideal be closely correlated with the effective fishing intens exerted in order to minimize and temporal variations in q. At the same ti~2, variations in q may result from changes in fishing power, effort distribution in relation to population density (Rothschild and Robson, 1972), as well as changes in availability by sex, size, and age. Corrections for these factors whenever possible should therefore be an integral part of fishing effort definition. The approach adopted here is to summarize for each type of gear what effort units seem most appropriate and the considerations that may lead to variations in q. Evidently, in order to sum the individual units of nominal effort exerted by members of the fleet, some account must be taken of their relative fishing power. Thus, the definition of Gulland (1964b) states that "The fishing effort of a fleet, from commercial statistics, is the sum of efforts of individual units, each computed as a product of fishing power of that unit and the time spent fishing, or number of We may note fter Saunders and Morgan (1976) that if gear efficiency (p) = cln, where c '" catch per unit operation from n individuals within the area of gear Ii

a c

a p, so that n the f intensity exerted by g units of nominal effort by a given vessel is f == gr. In , because of problems in absolute fishing power, it is usual to compare effective catches by different fishing units to a standard vessel or vessel type within the same time-area stratum to obtain re1a!ive fi power r' (Robson, 1966) before up corrected nominal effort units to obtain total fishing effort of the fleet as infl uence

f

the absolute fishing power of the gear (r)

n

f

r

L: i

where the summation is over the i individual vessels in a fleet of i

n units. In general, for all gear types, the conversion from nominal to effective effort should eliminate, where possible those factors which lead to variations in q, whether due to variations in power, gear configuration, or any factors that affect the additivity of the effort unit.

3

Application of Definitions of Fishing Effort and Assessments of Shellfish Stocks

in Population

A necessary SilllViiLi~a'~i'JH has been on the subject_ cons..L~''''"_..L_'''j the problem of effort definition in terms of four types of gear or methods fish 1. hand 2•

harvesters)

trawls 4. and set gear 3•

The main characteristics relevant to the for each of the above type of gear are reviewed under the sections.

of effort definition headings in

The uses to which improved estimates of effort and mortality can be applied are briefly reviewed here. T~2 first which is most relevant to the question of effort definition per se, may be considered under the heading of models [eg., Schaefer (1957); Pella and 'romlinson (1969); Fox (197:)); and other which status of the stock in terms of the relationbetween amount of fish effort and we of ca-tch. The different approaches are all encompassed within the Generalized Production model, express the in population size over time by: dP/dt = ±HPtm±KPt-qfP t where i the size, and H, K, and m are parameters that allow of a wide range of curves to the of overall catch on fishing effort. The general s is that catch rises with effort to some point (MSY) before with further increases. This treats the as a "black box" to which effort is the main input, and an estimate of Id at that level of effort the main output. it relative limited data is to errors or biasses in the units effort s both in fi power and gear well as in b of the stock. The other relies on to estimate mortal rates and sizes from size composition of the catch (catch curves) virtual population or cohort (Gulland, 1965; , 1972 1974). If an estimate of natural mortality rate is available, these allow estimation of mortality rates, and comparison of mortality rates with trends in effort, in the coefficient q can be detected with age and time. may be defined as the Ii of one individual in the chosen at random by one unit of effort, variations in q may be due to one or more of the factors: 1. changes in fishing power 2. in vulnerability,

r~TC>NlT,

or stock aggregation.

power are caused by stay on concentrations case of whose catch

)

o

the gear is -Lltl]JI",OJOJible i~he units shel fish

stock (Rothschild

, 1975).

at definition of effort in between search time time fishable stock) and time the time which in (Beinssen 1976b). Although it may be fisheries to distinguish these two within effort available from commercial statistics, in many tr . d search time b -le In ex handl tim~ may e

to increase

as stock

HAND GA'rHERING

of largely wide (Medcof, 1961), cockle and clam (MacPhail and various fisheries with the aid of rmi manual collection of subtidal shell197Gb) . be considered

ty

s pc which may o tine (as in some in an elemen here since

do not at first ion gear se of conscious judgement (hand cuI ), or clam hacks) or be on sieve mesh size cockle fisheries). These types of fisheries present certain which may be conveniently presented to a or lesser extent to more elaborate fisheries. LU_-L£JU L

Effort De While of effort units may best be in terms of manon the qrcnuJds, hours underwater by divers, or directly in terms of the area of terrain searched, a definition of the between effort and fish take into account several additional factors

binomial _, 1974) seems wide of this of (Gulland effort statistics subunit areas. necessary to assess each unit area fishing and

Pielou 1965 may confound a unit of effort exerted at any Many otherwise) share with benthic sms liott 1971), and fish to 1955) is stratification of the catch and may then be over the whole

2) The time spent , for it would seem 1 that the resultant effort distribution will also be non~random. In addition, there may be a distinct cut-off terms of the minimum CPUE 'that may precede for another more part of the stock.

3) The definition of a unit stock posed one of the or problems for which are at least subtidal, a fraction of the stock may be available to ion. The efficient nature of hand for such as Abalone spp.) and large decapods may result in the extension of the range of a into progress distant or inaccessible waters (reefs, etc.) with the result that on stock depletion, effort units measured in or hours at sea may come to consist of search time as opposed to time the catch. indirect indices of time spent volume of gases used per trip, may overestimate actual time spent since and inaccessibility of harvestable densities may of estimates a , two types

to the may be

disLi clam d out" of cockles (Franklin, 1972), fisheries (eg. Dare, 1974 , 1973) mortalities are to make based on numbers of individuals landed (by is) to underestimates of Catch and is may the most tractable to the estimation of the re between f effort and sustained for these fisheries;

MacPhail, 1964), and some types of where indirect

2. other (often efficient) s of hand catch statistics may an alternative to such methods of virtual CU1CU.V'"is •

good

DREDGES AND MECHANICAL HARVESTERS

terms )

and seal without, upper and lateral As a special

Se

and Catchab

,

clams) , 1959) or with (P'''>llrno, 1964). the continuous

Coefficient

are relative or sampling devices for benthic or 'thic v,cyall~SmS (Holme, 1964; 1 1956); and odometers have been used to measure distance travelied by dredges on bottom, the same studies (B'JurD,e, 1965) have indicated that mesh selection is poor if the are towed to fullness, or if other debris blocks the in and Gibson, 1956). Under these circumstances correlation between selection factor and volume of contents may occur as noted by McCracken (1963) for otter trawls, but to a more extent. As a result, the range of sizes partially retained the may extend over a wide of the available 72) , cull undersized individuals exerted of teeth reduce the amount of debris . obs), fraction of This must be if the terminal the bot'tom.

type may be

gear of and pern,af>s, is the effect of the stock referred to earlier, which means that definition of effort in terms of area swept by the (Baranov, 1918) may have to be modified in the light of the distribution the species ( , 1975; Allen, 1976). the

Seasonal factors such as weather may affect signif increased "j of the , even the addition of pressure to maintain contact with the bottom (Baird 1959). Gear may also vary on over the same ground particularly due to recess of the effects of the in mod and the bottom LUH!".Lll'1

Behavioral.

More active offshore behavior which can affect to some extent on

(Chlamys show active will

makes due to than close contact with the bottom, may exert selective indicated of individuals at size may be caused contact the gear. , for some has a size selective ; and H.L':)l"e::> t probasize at of seems to occur Another which i t is just from undercomponent of incidental mortal is that caused dumping of sized individuals from deck either due 1974) . the catch and cull , or Both factors may contribute to death dredges to once returned to the mesh screen used for many infaunal bivalves may 'rhe general in the delivery chute (Franklin, 1972) or manual destructive is that this type of gear is ) than manual undersized individuals of the fishing the of modification further harvest consideration (eg., deGroot and Another major problem the incidental damage that occurs in fisheries this moss rakes to other Possible incidental consideration. (eg., lobsters Scarratt 1975) may have an influence on effective F r. units in power calculations effort and be used to strati ratio of effective and effort summation. Crew size may influence carried out at sea. time where o the catch time over

Def

ion

Fisheries

can as at sea or on the be may introduce a dens bias to estimate of effort This because the ratio of search time and time to time and process the catch will both vary with abundance. More effort units may be either defined in terms of the time on bottom the gear which may then be converted into area (where area = gear time on bottom effective width). If the gear is unselective and saturated the number of tows the gear or gear of a known capac may be more measure of pressure. In this case of influence of the will be a function of and the of shellfish in the material retained the (Table 1).

8 -

THAW], FISHEHIES

, and trawl fisheries 957; 1960; 1974 discussed here, same gear.

capture of reptant: Considerations

Fishing Power Two groups of definition:

may pose

1) width and area swept may be the power, and

sl_~ ..

,_-.l

different

ems in effort

crustacea) where effective t.rawl factor in fishing

2) which may be the water column (eg., squids) or where least some movement off bottom occurs (eg., many commercial headline cross-sectional ) f area of the trawl mouth are in volume.

There are indications that effective trawl width not include the trawl wings if is not an consideration for burrowing species such as The use of the "area may lead errors in effective effort (Hoydal, 1976; Carlsson, 1976), because of loss of over the headline (which can vary ~~HL,.~_L) ) , but also because factor" by which fishermen power to some istics may be used measures of power correlated with

Factors

and Gear Selection

varies on bottom temperature content and Munch-Petersen, 1976) f and may also vary Behavior of males and females may be affected. Selection of trawls may be less clear cut than for groundfish s due to "'''';::''1.Lll,::!, and in many cases there is a s by-catch of small groundfish which has several gear that minimizes fish catch. Effort definition. The here are similar to those well defined for groundfish trawl fisheries. units of effort may be either in days spent on the ground, number of tows, or distance gear of a known type, width, or cross-sectional area or volume.

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Corrections to the Effort unit The of effort d(~fj nition in mult "Hl'l'cies fi,,!lerie;; has been addressed ('lsewhere (eq., Anon., 19()(); Ke j(). This mi:l.y tw particular important for those showinq contaqioufl e1istr ion