1. Assesment of Highly Migratory
fish Stock
Presented by:
Olik Jomang
M.F.Sc 1st year
Dept. FRM

2. Introduction
• The term highly migratory species (HMS) has its origins in
Article 64 of the United Nations Convention on the Law of the
Sea (UNCLOS). The Convention does not provide an
operational definition of the term, but in an annexure
(UNCLOS Annexure 1) it has lists the species considered highly
migratory by parties to the Convention.
• The list includes: tuna and tuna-like species (albacore, bluefin,
bigeye tuna, skipjack, yellowfin, blackfin, little tunny, southern
bluefin and bullet), pomfret, marlin, sailfish, swordfish, saury
and ocean going sharks, dolphins and other cetaceans
• This is a legal definition rather than a scientific definition
based on the actual migratory behaviour of the species.

3. • Some of these species may only occur and/or be caught
within EEZs but the available global database does not allow
distinguishing between catches made on the high seas and
those made within EEZs.
• Highly migratory species are therefore discussed without
regard to stocks or occurrence within EEZs or on the high seas,
except for tunas and some tuna-like species for which more
detailed data are available

5. • High sea fish stock produce 10% of the world food supply.
• These high trophic level oceanodromous species undertake
migrations of significant but variable distances across oceans
for feeding, often on forage fish, or reproduction, and also
have wide geographic distributions. Thus, these species are
found both inside the 200 mile exclusive economic zones and
in the high seas outside these zones.
• Highly migratory species can be compared with straddling
stock and transboundary stock. Straddling stock range both
within an EEZ as well as in the high seas. Transboundary stock
range in the EEZs of at least two countries. A stock can be
both transboundary and straddling.

6. General Assumption while Classical Stock Assessment
and Migratory fish stock Assessment
• Several methods presented so far are often insufficient when
applied to migratory or schooling fish stocks.
• General assumption :We can take representative random
samples of the stock.
• Migratory Species: Horizontal or vertical migration to areas
not covered by the fleet or normal fishing gears,it is usually
not possible to sample such stocks during these periods.
• Lead to gaps in the samples ,its is also possible that the
sample taken represent different parts of the stock and in
such case it is likely that the data may misinterpreted.

7. The concept and study of migration
• Harden Jones (1968) :Defined migration as class of movement which impels
migrants to return home to the region from which they started migrated or a
systematic type of movement of individuals belonging to a stock.
• He recognize three type of migratory movement:
1. Drifting with currents
2. Random locomotory movemnts and
3. Oriented locomotory movement.
• Dingle (1980): Has opined that accedental or unintentional movement can not be
included in the definition of migration.
• Random movement are not considered migration in context of stock assessment
because the interest of the subject is in type of migration that create bias for a
length frequency sampling programme.
• Predictatable e.g for some stock we can predict at which time and where high
concentration can be found.
• For stock assessment purposes , the explanation of why fish migrate is of little
importance .The important thing is to know where the fish are at which time of the
year.

8. Bias create by migration
1. Daily vertical migration(e.g at the bottom during day and in the water
column during night)
2. Daily horizontal migration e.g skipjack tuna has been observed to move
away and return to a precise location each day(Yuen,1970)
3. Spawning migration
4. Size dependent vertical migration of adults(skipjack tuna,the younger
year classes occur in surface schools,whereas the older specimens move
to greater depths)
5. Size dependent horizontal migration (i.e larger specimens move to
deeper waters ,while
6. Migration of juvenile,they may remain in the upper layers day and night
whereas the adults are at the bottom during the day

9. Fig:Illustration of Potential Bias due to migration during stock
assessment

10. • However, numerous HMS exhibit spatial and temporal
overlap, which creates management challenges since large
quantities of non target HMS are often caught incidentally.
• The various HMS captured may not have the same intrinsic
population growth rates or carrying capacities; therefore,
their populations may not exhibit the same responses to a
given level of fishing effort.
• Since fishers often seek productive stocks, non target
species may be depleted at a rate faster than target
species, thus sustainable management of all stocks may
require a reduction of effort well below that which
maximizes yield of the target species.

11. Bias when migration route is not
known
• Assumption:The migration patten is not known but believed
that the entire stock is on the fishing ground all year round.
• In this case ,while doing sampling for estimation of growth
parameters ,the important data for the first half year of the
life are not available.
• Then here one may wrongly explain the absence of small fish
as overexpliotation of the stock.
• And an obvious misinterpretation can be mortality

13. Study of Migration route
• Tagging: The classical way to study movement of fish is a taggging
programme.
• Identification tag are attached externaly or placed in the body
cavity ,the fish is measured and released at a known spot.If
successful,may provide a lot of useful information on the net
displacements between the point of release and the point of
recapture.
• Demerit:Do not provide information on what has happened in
between those moments and points.
• Sophisticated acoustic and radio tags allow continous observations
of the movemnt of single fish as followed from a research vessel.
• Latest models of tags record the compass bearing and tilt angle of
an individual fish

14. • Pop-up satellite archival tags (PSATs) is an archival tag (or data
logger) that is equipped with a means to transmit the
collected data via satellite.
• Its major advantage is that it does not have to be physically
retrieved like an archival tag for the data to be available
making it a viable, fishery independent tool for animal
behavior studies.
• They have been used to track movements of ocean sunfish,
marlin, blue sharks, bluefin tuna, swordfish and sea turtles.

15. • The Annual-Return Matched Sample Methods
• Its is a method for the estimation of growth parameters and
mortality rates of migratory stocks.
• The method is based on the assumption that a fish stock
follows a predictable migration route.If this migration route is
known (e.g from taggings) in the time and space we are in a
position to follow the cohort and to match samples from the
same cohort.

16. Hyphothetical Model
Area B(summer) Area C (Autumn)
Area A (Spring) Area D(Winter)
A,B,C,D symbolize geographical area.We assume the stock to
undertake the same migration each year.

17. General Matched Samples Method
• This method assumes that we have knowledge or a
hypothesis of the migration route in time and space , and
therefore are able to match samples so that they originate
(or can be hypothesized to originate ) from the same
cohort.
• To illustrate the feature of the general matched samples
method for estimation of total mortalities a simple
hypothetical example was conducted.This simplified
example deals with one cohort migrating through three
area,A,B, and C .
• From A the cohort moves to B and then to C where its stays
for a while ,its moves back to B where it started in A

18. CA
B

19. Case study
• Seasonally migrating species sometimes migrate earlier in the season, the
older and bigger fish first.
• The problem is analysed in well documented studies of a stcok of North
Atlantic mackerel,Scomber scomrus.
• The size dependent migration of this stock has been demonastrated by
Darson(1986) and Eltink (1987)
• The stock lives in north and west of the United Kingdom and Ireland
• The main migration route :The stock undertake annual migration from the
“over-wintering area “ to the spawning area “ , from spawning area to
feeding area and from feeding area back to the over –wintering area
• The total distance travel:500-1000 nautical miles per year.
• Spawning: March to july
• Sampling: Monthly basis and age,length and maturity stages were
recorded.

21. • Scomber scombrus is a species of temperate water thus
relatively easy to read the age from the otoliths.
• The findings of Dawson (1986) and Eltink (1987) were based
on random samples caught by commercial as well as research
vessels on the spawning ground during spawning period.

23. • The migration from the over wintering area to
spawning was found found to be occur in an
age size succession in which the old fish arrive
before young .

24. • Fig 11.6.3 (from Dawson,1986) illustrates bias problems when
estimating growth parameters for a migratory stock.
• The five growth curves were each estimated by the matched
sample methods.
• The data from the beginning of the spawning period (March)
produce a curve for large slow-growing larger fish compared
to the data collected at the end of he spawning period ,which
give a sleeper curve with a smaller L infinity.

26. Conclusion
• In marine ecosystems, highly migratory species (HMS) are
characterized as having vast geographical distributions, with
extensive individual migrations often spanning entire oceans.
Dispersal on this scale can promote ocean-wide population
connectivity, resulting in many HMS exhibiting genetic
homogeneity.
• From a biological perspective, these species often comprise a
single unit stock within an ocean basin. Since single stocks can
be distributed throughout multinational and international
waters (as with the tunas [Family Scombridae]), sustainable
management of these harvested stocks requires cooperation
between all fishing nations

27. Reference
• Dawson.1986.estimation of growth parameters of highly
migratory fish stock of Scomber scomrus of United kingdom
and ireland.
• FAO .1994.World review of highly migratory species and
straddling stocks, FAO Fisheries Technical paper 337, Rome
• FAO .2002. conservation and management of shared fish
stock, FAO Fisheries Technical Paper 465,rome
• FAO.2003.Fish Stock Assessment Manual,FAO Fisheries
Technical Paper 393,Rome
• FAO .2006. The state of world highly migratory, straddling and
other high seas fishery resources and associated species, FAO
fisheries Technical Paper 495,Rome