India has a coast line of about 8129 km. Fishing is largely in the hands of a number

fishermen and fishing is done by small mechanized and indigenous craft. Landings take

place almost all along in the coast line throughout the day and sometimes during

night. There are about 2000 fishing villages scattered along the coast line from where

fishermen go for fishing and return to a landing centre which may be distinct from the

fishing village. There are about 1300 landing centres scattered along the coastline of

the main land. Under these conditions collection of statistics by complete enumeration

would involve a very large number of enumerators and a huge sum of money apart

from the time involved in collection of data. In this context a feasible solution for obtaining

marine fish landings is the adoption of a suitable sampling technique for the collection of

fish landing data

Balan, K

CMFRI Digital Repository

CMFRI – Winter School on Ecosystem Based Management of Marine Fisheries Page 1 of 200 CMFRI Winter School on                                                                  Towards Ecosystem Based Management of Marine Fisheries – Building Mass Balance Trophic and Simulation Models      Technical Notes       Compiled and Edited by  Dr. K.S. Mohamed, Director, Winter School & Senior Scientist, Central Marine Fisheries Research Institute [CMFRI], PO Box 1603, Cochin – 682018, Kerala ksmohamed@vsnl.com CMFRI – Winter School on Ecosystem Based Management of Marine Fisheries Page 64 of 200     SAMPLING DESIGN FOR ESTIMATION OF MARINE FISH LANDINGS K.BALAN Central Marine Fisheries Research Institute, Cochin, India         India has   a coast line of about 8129 km.  Fishing is largely in the hands of a number fishermen and fishing is done by small mechanized   and indigenous   craft. Landings  take  place  almost  all  along  in  the coast line  throughout  the  day  and  sometimes  during  night. There are about 2000  fishing  villages    scattered  along  the  coast line from  where  fishermen  go  for  fishing and return to a landing centre which may be distinct from  the  fishing  village.  There  are  about  1300  landing  centres  scattered  along  the coastline  of the main  land.  Under  these  conditions collection of statistics by complete enumeration would  involve  a  very large number of enumerators and a  huge  sum  of  money apart from the time involved in collection of data. In this context a feasible solution for obtaining marine fish landings is   the adoption of a suitable sampling technique for the collection of fish landing data.           To  evolve a suitable sampling design, pilot  surveys  were  organised by different organizations in India from time to  time.  The design of the present survey has been evolved on the basis of  results  obtained in these pilot surveys undertaken from time  to  time.            The sampling design adopted by the Central Marine Fisheries Research Institute (CMFRI) is based on  stratified  multi-stage  random  sampling technique, the stratification being over space and  time.  The  stratification is made by dividing each maritime state  into  several zones on the basis of fishing practices and  geographical  considerations. The  number of centres included vary from zone to zone depending on the topography. These zones have been  further  stratified  into  substrata,  on the basis of intensity of fishing in a centre.  There  are  zones  which have only one centre, usually known  as  Single  Centre Zone.          One  zone  and a calendar month are taken as the  basis  of  space-time  stratum. If in a zone, there are 20 landing  centres,  there will be 20 x 30 = 600 landing centre days in that zone  for  that  month ( of 30 days). For observation purpose, a  month  is  divided  into  3 groups, each of 10 days. From  each  group, a cluster of  6  consecutive   days  are  selected  systematically with a random start with a sampling interval of ten days: Thus fom  the first five days of a  month,  a  day  is  selected  randomly,  which together with the next  5  consecutive  days  (6  days in all) form the first cluster. The next  6  days  each from the other groups follow systematically. For  example,  if  for a zone, the observation starts  from the 4th of a  month  and  continue upto 9th, the next cluster  will  start  from  the  14th and the last cluster from 24th. As the  days  are  selected  as per the above procedure, three centres are  randomly  selected for observations over 6 days and each selected centre is  observed  for two consecutive days. The observation is made  from  1200  hrs to 1800 hrs on the first day and from 0600 hrs to  1200  hrs  on  the second day, in a centre. The intervening  period  of  these two days ie, data collected by enquiry from 1800 hrs of the  7 CMFRI – Winter School on Ecosystem Based Management of Marine Fisheries Page 65 of 200 first  day  of  observation  to  0600  hrs  of  the  2nd  day  of  observation of a landing centre-day is termed as 'night landing '.            The  `night landing' obtained by enquiry on the second  day  covering  the period of 1800 hrs of the first day to 0600 hrs  of  the next day are added to the day landings so as to arrive at the  landings  for  one  day  (24 hours). Thus in  a  10  day  period,  data from 3 centre-days are sampled and consequently,in a month 9  landing centre-days are sampled .         Selection of units and recording of landings          It may not be practicable to record the catches of all boats  landed during an observation period, if the number of boats/craft  is  large. A sampling of the boats/craft become  essential.  When  the  total  number  of  boats landed is 15  or  less,  the  total  landings from all the boats are enumerated for catch  composition  and other particulars. When the total number of boats exceeds 15,  the  following  procedure  is followed to sample  the  number  of  boats :    Number of units landed                  Fraction to be examined    Less than or equal to 15                       100 % Between 16 and 19                               First 10 and the balance  50 %   Between 20 and 29                              1 in 2  Between 30 and 39                              1 in 3  Between 40 and 49                              1 in 4  Between 50 and 59                              1 in 5  and so on                        From  the  boats,  the catches  are  normally  removed  in  baskets  of standard size. The weight of fish contained in  these  baskets  being  known,  the weight of  fish in  each  boat  under  observation is obtained.    Estimation  procedure:                    From  the  landings of the observed fishing  units,  the  landings  for all the units landed during the observation  period  are estimated.  By adding the quantities landed during the two 6- hours periods and during the night (12-hours) the quantity landed  for  a day (24-hours) at a centre that is the landings  for  each  centre day included in the sample is estimated.  From these, the monthly zonal landings are obtained.     ?YnYijkNijkijklin???1  where Yijk is the estimated landings for the k th month in the  j  th  zone  of  the i th state, Nijk is  the   number  of  landings  centre-days in the k th month for the respective zone, n  is  the  corresponding number of centre days actually sampled and Yijkl is  the estimated yield for the l th landing centre-day in the sample  for the respective space-time stratum.                 However,  important centres such as Veraval,  Sassoon  Dock,  Sakthikulangara etc. larger sampling coverage is  ensured  as they are treated as Single Centre Zones.  CMFRI – Winter School on Ecosystem Based Management of Marine Fisheries Page 66 of 200           From the zonal estimates, districtwise, statewise and all India landings are arrived.    The corresponding sampling errors are also estimated.   Determination of overall sample size and variance in different strata/states:              The  sampling fractions at the level of the first  stage  units vary from 1.5 to 3% in different states.  With the  present  sample size, the error in the estimated total annual landings  in  India  is  about 5%.  The sample size allocated to  each  stratum  (zone)  is the same.  As the variance is found to  increase  with  the size of the landings, the good landing centres are given more  chances  of being included in the sample.  This is done  by  sub- stratifying  the  landing centres of a zone on the basis  of  the  intensity of landings.  A statewise allocation of the sample size  on the basis of the mean annual landings is found to be practical  and close to the optimum allocation.      Observational errors their magnitude and control:             The  estimated zonal landings are always compared   with  the  previous year's survey figures,and if any discrepancy  which  cannot    be   explained   is   observed,   the   technique    of  interpenetrating  sub-samples is adopted to detect  observational  errors.   Observational  errors  are rarely  confirmed  and  when  confirmed,  the  field  staff  is  either  called  back  to   the  headquarters for giving intensive training or he is replaced.  Errors due to non-response, their magnitude and control:            Non-response occurs only when the regular field staff is  not  available to observe the centre-day included in the  sample.   Usually,  arrangements are made to substitute the regular one  by  another on such occasions.    Sampling errors, methods for its estimation, statewise  magnitude  and control:             The sampling errors involved in sub-sampling the centre- day and the boats (units) are assumed to be negligible (Sukhathme  et  al.,  1958).  Therefore an estimate of the  variance  of  the  estimated  landings (Yijk) of the k th month in the j th zone  of  the  i th state ignoring the finite population correction  factor  is given by                ?V ? ?Y Nn Vijkijkijk?2      where  ? ?Vy ynijkijkl ijkllnln????????????? 21211  The  percentage  errors at the states level vary from 5  to  20%.   To  control the error, the stratification (based on the intensity  of landings) is made from time to time.        CMFRI – Winter School on Ecosystem Based Management of Marine Fisheries Page 67 of 200 Plan of operation:             The survey staff immediately after recruitment undergoes  a  training course which lasts 10-12 weeks and is posted  to  the  survey centres.  Each survey centre is housed in 1-2 room  rented  apartment  and  each  centre  is  provided  with  the   necessary  literature connected with the identification of fish, a reference  collection of local fishes, crustaceans and molluscs, field note- books and registers.  At the end of every month, the survey staff  receives  by post, the programme of work for the following  month  which  includes  the names of landing centres to be  visited  and  details  such  as date and time of observations at  each  landing  centre.  The programme is carefully designed at the  headquarters  by  the  statistical  staff.   The  field  staff  send  the  data  collected  during a month to the Institute's  Headquarter  before  the end of the first week of the subsequent month where, they are  scrutinised and processed by the statistical staff.    Supervision of scrutiny:             Surprise   inspections  are  carried  out  at   frequent  intervals  by  the  supervisory staff of the  Institute  and  the  enumerators  are inspected while at work in the field and  their  field  note-books and diaries are scrutinised and  initialed.   A survey staff is usually inspected twice in a month.   The data received at the Headquarter are scrutinised carefully at every stage for finding it in order.  The processed data are counter- checked for errors if any in the method of estimation.   Usually only cross checks are made; but when discrepancies are detected the estimation procedures for any zone are scrutinized in detail.    Tabulation and processing             Different schedules are used to collect fishery survey data. Necessary coding of species were also done to process the data with the help of computer facilities at Headquarters.                       

Sampling design for estimation of marine fish landings- Winter School on Towards Ecosystem Based Management of Marine Fisheries – Building Mass Balance Trophic and Simulation Models

Sampling design for estimation of marine fish landings- Winter School on Towards Ecosystem Based Management of Marine Fisheries – Building Mass Balance Trophic and Simulation Models

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