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BI 3063 J. Mork H10
Genetic and biologic stock management
The Northeast Arctic CodThe Northeast Arctic Cod
(N E A C)(N E A C)
biology biology stock structure stock structure
exploitation exploitation management historymanagement history
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BI 3063 J. Mork H10
Genetic and biologic stock management
In their Chapter 5 on NEAC, Nakken et al. (2008) summarizes and evaluate the available information on trends and fluctuations throughout the 20th century in:
• fisheries• stock size • biological characteristics
They describe the development of our understanding of these fluctuations as well as the management measures which have been adviced and introduced in order to maintain the yield from the stock at a high and sustainable level.
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BI 3063 J. Mork H10
Genetic and biologic stock management
The Migrations of the NEAC ("Skrei") according to Sars (1879)
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BI 3063 J. Mork H10
Genetic and biologic stock management
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BI 3063 J. Mork H10
Genetic and biologic stock management
The life cycles of cod stocks which show temporal stability (i.e. at least for hundreds of generations), typically show some relation to current systems.
In the northeast Atlantic, particularly along the Norwegian coast, the "Gulf stream", the Baltic rest-current, and the Norwegian coastal current all contribute in carrying pelagic organisms northwards. The eggs, larvae and small codlings are all pelagic. They are carried northwards from the spawning areas, which are typically to the south of the nursery areas. When the gonads mature prior to spawning, cod individuals are triggered to a contranatant swimming behaviour which brings them southwards along the Norwegian coast, and so compensates for the northwards transport of the pelagic stages. Hence the full life cycle circle is closed.
Ocean currents which are stable over long time periods (like the "Gulf stream"), allow fish stocks like the NEAC to adapt genetically to a life cycle which utilizes the benefits of free transport to nursery areas and stable migration patterns. For NEAC this scheme has developed in ~ 10.000 Y.
THE IMPORTANCE OF STABLE CURRENT SYSTEMS FOR PERMANENT THE IMPORTANCE OF STABLE CURRENT SYSTEMS FOR PERMANENT LIFE CYCLESLIFE CYCLES
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BI 3063 J. Mork H10
Genetic and biologic stock management
Transport by ocean currents:Transport by ocean currents:
a simulation modela simulation model(Vikebø et al.)(Vikebø et al.)
(link)(link)
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0 5 10 15 20 25 30 35 40 4563
65
67
69
71
73
75
77
79
Longitude
Latit
ude
From Vikebø et al.
Results from computer simulationsof particle drift with currents alongthe coast of north Norway.
Blue trajectories represent verylight particles (high in the watercolumn) and red trajectories markvery heavy particles.
Blue and red particles are proxiesfor "typical" eggs of NEAC andNCC, respectively. In reality, the density of the two egg types showconsiderable ovelap.
NB! Intermediate weigth particleNB! Intermediate weigth particletrajectories are not shown. Their trajectories are not shown. Their routes would overlap extensively with routes would overlap extensively with red and blue trajectories in the figure.red and blue trajectories in the figure.
Genetic and biologic stock management
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BI 3063 J. Mork H10
Genetic and biologic stock management
THE NEACTHE NEAC
A STORY OF FLUCTUATIONSA STORY OF FLUCTUATIONS
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BI 3063 J. Mork H10
Genetic and biologic stock management
Upper graph: Notice that landings have increased relative to the corresponding stock size in the 20th century.
Middle graph: With some exceptions (e.g. during World war II), the fishing mortality has increased steadily during the 20th century.
Bottom graph: Notice the high annual variability in year-class strengts, and the tendency of periodicity in strengths.
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BI 3063 J. Mork H10
Genetic and biologic stock management
Norwegian are not the only ones to exploit the NEAC. In particular Russia, but previously also England/Scotland, have harvested substantial mounts from this cod stock. While much of the Norwegian fishery targets the spawning cod with gear like gillnets, handlines, bottom lines and Danish Seine in Norwegian territorial waters (e.g. Lofoten), countries like England, Faroes and Iceland often use trawl gear in the Barents Sea outside the spawning season.
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BI 3063 J. Mork H10
Genetic and biologic stock management
Modern trawling rocketed from the 1930ies except during the WW2, much due to the availability of efficient combustion motors for the fishing fleet. The shift from pre-WW1 mainly spawning fishery by Norwegians (gill nets, handlines and longlines), to the Barents Sea trawling by an international fleet is very evident in the graph. The preceeding Table 5.3 shows that this coinsides with very marked increases in the total landings.
The connection between the gear used and the total landingsThe connection between the gear used and the total landings
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BI 3063 J. Mork H10
Genetic and biologic stock management
Except for a short burst in the "overly optimistic" 1950ies, purse seine have not been employed in the Norwegian spawning fisheries. Since 1950 gillnets have been the most significant gear used in the NEAC spawning fishery.
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BI 3063 J. Mork H10
Genetic and biologic stock management
The Lofoten spawning grounds are located in Nordland County, which stands for most of the annual landing. Troms and Finnmark Counties have been number since 1935. Before the 1930ies, the traditional "skrei" fishery in Møre and Romsdal County contributed comparatively more to the total. The "skrei" spawning in those more southern southern areas have ceased considerably over time.
Proximity to the resource dictates larger NEAC landings in Proximity to the resource dictates larger NEAC landings in the northernmost part of Norwaythe northernmost part of Norway
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BI 3063 J. Mork H10
Genetic and biologic stock management
Fig. 5.9 shows the spawning migrations of cod tagged in 1912-13 on the Finnmark coast to two distinct spawning grounds; the Lofoten and the Møre area. The importance of the Møre area is much lower today. Fig. 5.8 shows the northward drift of cod eggs and larvae.
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BI 3063 J. Mork H10
Genetic and biologic stock management
The mean length for recruit spawners has increased for age groups 8-10 years after WW2. The "dip" during WW2 is believed to be caused by a complete disappeance of spawning capelin off the Finnmark coast in 1938-1942.
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BI 3063 J. Mork H10
Genetic and biologic stock management
A "dip" similar to that observed during WW2 was was observed in the late 1980ies. Again, bad nutririon conditions were probably the cause. A simultaneous failing of the prey species capelin and herring is dramatic for the NEAC (as well as for harp seals and seabirds.
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BI 3063 J. Mork H10
Genetic and biologic stock management
Maturation age for NEAC has shown large variation over time, with an overall downwards tendency (Fig. 5.12)). The reasons for this trend have been explored; candidate explanations are temperature increase, heavy exploitation (less individuals and more food for each), and genetic effects (heritable changes in the gene pool) by using size-selecting gear.
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BI 3063 J. Mork H10
Genetic and biologic stock management
Causes of fluctuation in recruitment; environment or spawning stock size?Causes of fluctuation in recruitment; environment or spawning stock size?
The (wrong) view that fishery had no significant effect on stock recruitment was held by most scientists until the 1960ies! This view caused catastrophic events like the crash of the Norwegian Spring-Spawning herring stock. In NEAC, the current view is that of the Russian Ponomarenko (1973): "A large spawning stock comprising many age groups will spawn over a larger area and a longer time than a small spawning stock, so that it becomes more likely that some
parts of the eggs and larvae from a large spawning will encounter favourable conditions for survival".
Effect of spawning stock size
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BI 3063 J. Mork H10
Genetic and biologic stock management
NB! The regression line is wrong in this figure.
The plots of age=3 recruits against water temperatures during early development clearly shows that high temperatures is favourable for survival and recruitment to the NEAC.
Effect of water temperature
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BI 3063 J. Mork H10
Genetic and biologic stock management
Effect of suitable food(depending on temp.)
A clear relationship between peak occurrence of the main food item Calanus and sea temperatures has been established in northern waters. Cod larve depend on the availability of Calanus during their early life stages in April. This dependency may be the cause for the observations behind the the relationship in Fig. 5.14 (former slide).
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BI 3063 J. Mork H10
Genetic and biologic stock management
The North Sea – Barents Sea recruitment paradox explainedThe North Sea – Barents Sea recruitment paradox explained
It was early observed that high water temperatures were favourable for cod in northern waters but not for North Sea cod. The explanation to this apparent contradiction was suggested by Sundby (2000): Differences in the transport of the copepod Calanus from their core areas in the Atlantic to the southern and northern cod habitates in warm and cold years. Again; differences in the availability of suitable food organisms.
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BI 3063 J. Mork H10
Genetic and biologic stock management
The spawning stock – recruitment relationshipThe spawning stock – recruitment relationship
It was only after 1950 that the data and modern analytical tools for the analysis of stock- recruitment relationships became available. By then, the development of the theory of fish population dynamics and its implementation using catch-at-age data changed the situation. Today it is generally acknowledged that a large spawning stock consisting of a high proportion of "old" (second and third time) female spawners is the most favourable situation for high recruitment rates. The eggs of old females have larger yolk sacs and are hence better nourished. Furthermore, they show greater variation in buoyancy and will be more dispersed vertically and horizontally so that hatching is spread over a larger geographic area and survival is increased.
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BI 3063 J. Mork H10
Genetic and biologic stock management
The importance and utility of stock monitoringThe importance and utility of stock monitoring
The NEAC year-class stock size was monitored by Oscar Sund, using length distribution in catches as a proxy. Strong and weak yearclasse stand out as deviations from the mean and can be followed over time as they set their "footprint" on the catches.
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BI 3063 J. Mork H10
Genetic and biologic stock management
"Modern" recruitment surveys; the acoustic "echo-integration" methodology developes"Modern" recruitment surveys; the acoustic "echo-integration" methodology developes
Abundance of fish estimated in different ways: Acoustic indices, Swept area indices, and Virtual Population Analysis (VPA) based on catch at age data.
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BI 3063 J. Mork H10
Genetic and biologic stock management
TACTACMany lessons have been learned in the last century, about the uncertainties connected with various methods for stock abundance.
For example, the value of the CPUE (Catch Per Unit of Effort) as a proxy for stock size can fail completely if the stock for some reason (e.g. temperature anomalies) change distribution area and/or beomes more concentrated on short terms.
Most failures done has been in the categories "stock overestimation" and "exploitation underestimatiin", often occurring at the same time. In addition, politicians tend to "give more" to fishermen organisations than recommended by fishery scientists, for political reasons.
