sh_vol16_03_1972.pdf (1.118Mb)

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F I S M E R I B I R E K T O R A T ' E T S S M R I F T E R

S E R l E M A V U N D E R S O K E L S E R R

D I R E C T O R A T E O F F I S H E R I E S YIERGE?,N3 N O R W A Y

I 9 7 2

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FiskDir. Skr. Ser. IlnvUtzders., 16: 77-88.

THE RELATION BETWEEN THICKNESS OF CHORION AND SPECIFIC GRAVITY OF EGGS FROM NOKWEGIAN

AND BALTIC FLATFISH POPULATIONS

By

SUNNIVA L ~ N N I N G

Institute of Physiology, University of Bergen, Norway and

PER SOLEMDAL

Institute of Marine Research, Directorate of Fisheries, Bergen, Norway

A B S T R A C T

L ~ N N I N G , S. and SOLEMDAL, P. 1972. The relation between thickness of chorion and specific gravity of eggs from Norwegian and Baltic flatfish populations. FiskDir.

Skr. Ser. HauUtlders., 16: 77-87.

Neutral buoyancy, size, and thickness and ultrastructure of the chorion were studied in eggs from populations of flatfish (Plnfirhthy~ j'lesus and Limanda limanda) from western Norway and three localities in the Baltic: Kiel, the Arkona basin and Tvar- minne. The observed differences in neutral buoyancy are probably caused by the dif- fering thickness of the chorion and are correlated with the salinity in the area. The differences are thought to be the result of a long term selection process.

I N T R O D U C T I O N

Marine teleost species with pelagic eggs are often distributed over a wide range of salinities (APSTEIN 1910, KHNDLER 1941). Brackish water populations of such species produce eggs of a lower specific gravity, capable of floating in water of reduced salinity. Usually, but not in- variably (see e.g. MIELCK and KUNNE 1932), such eggs are bigger than those of marine populations of the same species.

Most authors have explained the low specific gravity of pelagic eggs in brackish waters as a function of the osmotic conditions, either in the ovary (STRODTMAN 1918, SOLEMDAL 1967), or in the ambient medium

(JACOBSEN and JOHANSEN 1908, KHNDLER and TAN 1965, HOHENDORF 1968). T h e immediate osmotic effect on the specific gravity is, however, small, and can only explain part of the observed differences between the eggs of marine and brackish water populations. I t has therefore been suggested that the existence of pelagic eggs with low specific gravity in brackish water populations is the result of long term selection (SOLEMDAL 1971).

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Interspecific differences in the thickliess ancl ultrastructure of the chorion of pelagic teleost eggs have been described (GOTTING 1966, HAGSTROM and LONNING 1968, LONNING 1972). As the chorion is the

5 O l o 0 15' 20° 25O 30°

Fig. 1. Sampling localities.

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densest part of the egg, such changes have a marked influence on the specific gravity of the egg. T o test the hypothesis that intraspecific variation in the chorion between marine and brackish water populations of a given species may be responsible for the observed differences in specific gravity, eggs from flounders [Platichthys .flesus ( L I X N A E U S ) ~ ancf dabs [Limanda limanda (LINNAEUS)] caught a t Bergen and a t different localities in the Baltic have been investigated with regarcl to size, neutral buoyancy, and thickness and ultrastructure of the chorion.

M A T E R I A L AND M E T H O D S

Flounders (Platiclztlys j'lesus) and dabs (Limandn linzanda) were caught near Bergen in water of 35 0/,, salinity ancl from Kiel Bay, 22 ^O/,, S.

Flounders were also collected from the Arkona basin, 17-18 ^0/,, S, and froin the area near Tvarminne Zoological Station, 6 O / , , S (Fig. 1).

Diameter and neutral buoyancy were deterininecl on ilormally de- veloping eggs 24 hours after fertilization,using the methods described by SOLEMDAL (1967).

The fixation and embedding procedure for electron microscopy has been described by LONNING (1972). The material was sectioned on a LKB ultrotome I11 and examined in a Siemens Elmiskop I.

Ultrastructural studies were mainly carried out on eggs fixed 3-5 days after fertilization, and wheil nothing else is mentioned the micro- graphs are taken from this material. From the Bergen area unfertilized eggs and eggs fixed immediately after fertilization were also investigated.

R E S U L T S

Neutral buoyancy and egg clianieter have been measured for eggs of populations of Platiclztlysflesus and Lilnancla lima?zda, living uncler different salinity conditions, and the results are given in Table 1. From this i t can be seen that, in general, neutral buoyancy decreases with salinity, with the exception of the flounders from the most brackish area, Tvar- minne, which have eggs of somewhat higher neutral buoyancy than those from the Arkona basin. I n the Arkona basin the buoyancy is only just enough to keep the eggs from sinking, a t Tvgrminne flounder eggs sink and clevelop demersally. Similarly, egg diameter in both flounder and dab increases with decreasing salinity. Here again the sole exception are the flounders from the Tvarminne area, which have significantly smaller eggs than those from the Arkona basin.

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Table 1. Neutral buoyancy and diameter of flatfish eggs from marine and brackish populations.

I

Salinity Pelagic: P Egg diam. mm

Number

species Locality

0

1

^buoyancy" " 0

s

^Demersal^D

1

^Mean

¹

Limanda limanda Bergen 35 30.9 P 0.770 0.75-0.87 332 7

Kiel 22 17.5 P 0.928 0.84-1.04 485 7

Platichthys flesus Bergen 35 31.8 P 0.884 0.78-0.94 1 760 14

Kiel 22 - P 0.959 0.89-1.02 339 2

Arkona 17-18 16.2 P 1.185 1.01-1.29 1 470 14

Tvarminne 6 19.9 D 1.008 0.88-1.11 1 723 19

I

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I n electron microscopic studies the chorion of the fish egg is usually described as consisting of a thin outer layer and a thick, lamellar inner layer. The nonlenclature of these layers varies among authors; in this paper the purely descriptive terms <<outer layer)) and {(inner layer)) will be used.

Recent studies of the eggs of Platirhtly flesus and Lilna~zda limanda froin the Bergen area have shown that the chorion of the egg has about the same thickness in these t ~ v o species, viz. about 2.5 p ; the ultrastructure of the chorion, on the other hancl, sho~vs clear species-specific differences (LONNING 1972).

I n eggs of P. jlesus from Bergen the inner lamellar layer of the chorion consists of 6 lamellae and the distance between the lamellae varies, being largest between the micl-lamellae. Butsicle the lamellar part a reticular layer is present. The outer layer of the unfertilized egg is rather homogeneous whereas in the 3-5 days old larvae this layer is fenestrated or subdivided into several layers (Fig. 2, see also LBNNING

1972, figs. 10, 12).

Eggs of P. ~flerus froin Kiel Bay show a somewhat thinner chorion, viz. about 1.8 p. Also in these eggs the number of lanlellae is 6, but these lan~ellae seem to be more equidistant than in the Bergen material. The reticular layer and the outer layer are in these eggs more homogeneous and not so distinct (Fig. 3).

Eggs of P. flesur from the Arkona basin have a quite different chorion from those from the Bergen and IGel area. The diameter is only ca.

1.1 p and there are but three lamellae, of which the two outer are furthest apart. As in the Kiel material the outer and reticular layer are rather homogeneous, but in the Arkona population these layers are thinner (Fig. 4).

I n eggs from Tvgrminne, finally, the chorion is nearly as thick as in the Bergen material, viz. 2.3 p. T h e number of lamellae, however, is only 4. Outside the lameIIae a rather distinct, reticular layer is present, fol- lowed by the outer layer, which is often solnewhat subciivided (Fig. 5).

As mentioned above the eggs in this population are clemersal.

The chorion of the egg of Lima~zda limanda from the Bergen area has been described earlier (HAGSTROM and LONNING 1968, LONNING 1972).

As already mentioned the diameter is 2.5 p. I n this species the lamellar part consists of 9 equidistant lamellae and the outer layer is subclivicled into several homogeneous layers of different electron density (cf. Fig. 6, which shows an unfertilizecl egg).

Eggs of L. limanda from Kiel (Fig. 7) have a thinner chorion, viz.

1.6 p. The number of lamellae is 8 or 9, and also in this population the lamellae are equidistant. The outer layer is less homogeneous than in

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the Eergeri material, and is some~vhat broken up. As tlie Kiel material consisted of 5 clays oicl larvae, sollie of tl-icsc clifferences, especially in the outer layer, inay be clue to morphological ciianges during development (cf. L O ~ I N G 1972).

A synoptic comparison of the data on thc ultrastructure of the chorion i n tlie investigated pol~ulations of P. f l ~ s u s and L. limanda is given in Table 2.

Table 2. Dianletcr of cllorion and ~i~unlber of lamcllae of flatfish eggs from marine and brackisll popnlations.

Species

Platicl~tl~ys flesus Bergcn 2.5 6 13

ICiel 1.8 6 5

Arkona 1.1 3 5

Tvarminne 2.3 4 6

I I Chorion

I Limanda litnailria

'

^Bergol

1

^Kiel

Number of

The l o ~ v specific gravity of pelagic fish eggs in brackish areas is of survival value as it ~vill cause the eggs to float in water of lowered salinity.

Sliort term experiments .r\~ith flounclcrs which were transferred froin high to low salinity slio~uecl that the specific gravity of the eggs was little affected (SOLEMDAL 1967). Siniilarly, Baltic flcltfish kept for t ~ v o years in w2ter of full marinc saliiiily still procluced eggs of l o ~ v specific gravity (SOLEMDAL. in press). The differences 11; neutral buoyancy in pelagic eggs of ~ n a r i n e and brackish water po~ulations of the same species can thu.; not be explained by water absorption by the eggs clue to osmotic conclitio~is in tlie ovary or the surrouncliiig medium.

Of the diffcrcnt egg components, tlic chorion is by far the densest.

I n herring the chorion amounts to 15-30% of the total clry weight (BLAXTER ancl MEMPEI 1963), in plaice 19-- 33

%

(SOLEMDAL 1970).

Generally the Baltic populations have eggs with a thinner cl-iorion than tlie marine populations (Table 2). I n Platzchthys flerus, fishes from the Arltona basin have the thilincs~ chorion, while at Tvarniinne, in still

i ~ O c a l i r ~ Thickness {i

1

Number of eggs Mean lamellae rneas~~red

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more brackish water, the chorion is muclr thicker ancl has in Uact ncarly the same value as in the n~ariilc populations. H o ~ l e v e r , a t Tviirminnc the eggs develop at the bottom, so that a tliicli, tough chorion is a definite advantage.

Both thc thin chorion of the most brackish populations, and the thick chorion of tine Tvarminne flounders, iilay thus well be the result of a long term selection process This hypothesis is supported by the experi- nients mentioned above, ill wlzich it was s11owi1 that in Baltic flatfish kept a t high salinity for as mrrch as two years the eggs still ~ v e r e of the lotv gravity Baltic type. Differences in e.g. feecling conclitions, I~yclro- grapliy or spawniiig season in tlie Baltic populations can tllus be ruled out as a possible explanation for the differences in egg structure.

Pin botli Platiclzll!~~.rfles~~s aiicl Linzan(la lirnmzcla there is a n inverse correlation between cliaineter of the egg ancl its specific gravity (Table 1 ) . Such a correlation does not exist, on L B ~ other hand, in Pleu~ouectes fllniessa.

I n this species tlie Baltic populations have the sillaller cggs (SOLEMDAL, in prcss). Egg size per sc is thus not likcly to bc the 111aii1 cause of the observed differences in specific gravity.

A C K I Y O W L E D G E M E N T S

Thanks are due to the Biological Station, University of Bergen, 7 ' " . varrninne Zoological Station, University of Helsiiiki, professor Henipel and the crew of R.V. < < h l k o r ~ , Institute of Marine Research, Kiel, for working facilities and supply of ~ i ~ a t c r i a l s , and the Institute of Ana- tomy, Universit-y of Bergcn, for use of the elcctron n~icroscope. Thanks are also due to M r . Per Bratlailcl ailcl EtBrs. Marit Brustacl for technical assistance, and to Mrs. Peggy Q ~ ~ i s t - E a n s e n for correcting the English text.

O n e of the autl~ors (P.S.) is inclebtecl to the Norclic Council for Marine Biology for a visit to the rFviirinini~t Zoological Station.

R E F E R E N C E S

A ~ S T E I N , C . 1910. H a t eirr Organismus in der Ticle gelchi, in cler er gefischt ist? Int.

lieu. gcs. E-lyclmbiol. Iiril-ogl-., 3 : 17-33.

BLAXTEII, J. Pi. S. and &?RIPEL, G. 1966. Utilizatioll of yolk by herring larvae. ,7.

7110~. biol. As.r. U.K., 46: 219-234.

G ~ T T I N G , I<. J. 1966. Zur l'eii~strukiur cler Oocylell mariner 'Teleostecr. Helgolci~zder zaiss. r\%e~.esnizters., 13: 1 18-1 70.

H I ~ G S T I I ~ R I , B. E. a n d L ~ N N I X G , S. 1968. Elcctron microscopic studies of unfertilized ancl fertilized cggs from marine ielcosts. Solsin, 33: 73-80.

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HOHENDORF, K. 1968. Zur Schwebfahigkeit pelagischer Fischeier in der Ostsee. Be?.

dt. wiss. Konznz. MeetesJorsch., 19: 181-193.

JACOBSEN, J. P. ancl JOHANSEN, A. C. 1908. Remarks on the change in specific gravity of pelagic eggs. iVleddr Kotnintz Huuuizders. Ser. Fish-., 3(2) : 1-17.

K-YNDLER, R. 1941. Die Fortpflanzung cler Meeresfische in der Ostsee und ihre Be- ziehungen zum Fischeiertrag. iVIIz. Fisclz., 11 : 158-163.

KBNDLER, R. and TAN, E. 0. 1965. Investigations on the osmoregulation in pelagic eggs of gadoid and flatfishes in the Baltic. Part 1. Changes in volume and spec.

gravity at different salinities. C O Z ~ Z . Meet. int. Coz~tz. Explor. Sea, 1965 (43): 1-5.

[Mimeo.]

L ~ N N I N G , S. 1972. Comparative electron microscopic studies of teleostean eggs with special reference to the chorion. Sarsia 49: 41-48.

MIELCK, W. and KUNNE, C. 1932. Fischbrut und Plankton-Untersuchungen auf dem Reichsforschungsdampfer (tPoseidon)) in der Ostsee, Mai-Juni 193 1. Wiss.

Meeresutzters., Abt. Helgolatzd 3\f.F., 19 : 1-120.

SOLEMDAL, P. 1967. The effect of salinity on buoyancy, size ancl development of flounder eggs. Sarsia, 29: 43 1-442.

- 1970. Variations in physical properties of pelagic eggs in different populations

- genetic aspects. Cbzltz. Meet. int. COZLIZ. E,uplo~. Sea, 1970 ( F 29) : 1-3. [Mimeo.]

- 1971. Prespawning flounders transferred to clifferent salinities ancl the effect on their eggs. Vie A!Iiliezl, Sz$pl. 22: 409-423.

- Transfer of Baltic flatfish to marine environment and the long term effects on reproduction. Oikos. (in press).

STRODTMAN, S. 1918. Weitere Untersuchungen iiber Ostseefische. Wiss. Meereszltzters., Abt. Helgolnnd X.F., 14: 33-95.

Received 15 February 1972 Printed 12 December 1972

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Fig. 2 . Chorion w ~ t h outer Iayer (0) and inner Iayer consisting of a reticular part (R) and 6 lamellae ( L ) . 30 000s. Plnt~chtllys flecus from thr Bergen area.

Fig. 3. P. j'lesz~s from Kiel Ray. 30 000s.

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Fig. 4. 1'. ,/l'leslls from the Arlrona basin. 30 000s.

Fig. 5. I-'. ,/le.sus f ~ thc ~ ml'vii~.miiiiie area. 30 000x.

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Fig. 6 . Linzrzlln'cr linzcr~ztlr~ fro121 the Bergcn area. 30 0002.

Fig. 7 . L. linzcc/tclcl Irom I<iel Ba)-. .?0 0 0 0 ~ .

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FiskDir. Sky. Ser. Irlnuljizders., IG ^:59-87.

NOTES ON THE GAS CONTENT A N D NEUTRAL BUOYANCY IN PWYSOSTOME FISH

By

GUNNAR SUNDNES and PER BRATLAND

Institute of Marine Research, Bergen

A B S T R A C T

SUNDNES, G. and BRATLAND, P. 1972. Notes on the gas content ancl neutral buoyancy in physostome fish. FiskDir. Skr. Ser. HnuUtzdct.r., 16 : 89-97.

Field and experimental observations from herring (Clz@e(l hnrengnls) and char (Snlmo (I@~?ZZIS) indicate that the swimbladder gas is not only ctswallowed)~ from the atmosphere, but may also be depositecl from the vascular system or tissue around the swimblaclcler lumen.

I N T R O D U C T I O N

T h e genera Salino and Clupea are both physostoines and do not possess any organ or rete formations similar to those of the physoclists (FAI-ILEN 1967, 1971).

The ethology of the two species is fairly sinlilar. They are schooling fish which live in deeper water during feeding periods but move to shallower areas to spawn. During migration to shallower areas the herring undergo a vertical inigratioil resulting in reduced hydrostatic pressure.

T h e herring avoid overflotation by releasing gas. Prior to the development of acousticaI instruments, one of the methods of herring detection in the spawning areas was the observation of the numerous gas bubbles coming to the surface.

T h e char gas release is of a more con~plicated nature. The char do not only release gas to compensate for a decreasing hydrostatic pressure.

They also release gas by emotional stimuli when the fish are in a steady state of hydrostatic condition (SUNDNES, unpublished observations).

A similar behaviour has been described in the European minnow (DYKGRAAF 1932) and in some North American physostoine fish (VER-

HEIJEN 1962).

T h e gas filling mechanism for herring and char is not known in detail.

However, Clupeids are able to swallow gas at the surface (SRIVASTAVA 1964, FAHLEN 1967) and a similar function was described also for Sal- monids (JACOBS 1934).

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0 1 2 the other hand, I"i/ii1'rTEN~~R~ (1958) has describecl the refilling of eniptiecl swiniblac!ders of Salmonids in 13 clays due 1-0 secretion.

FAHLEN (1971) was not able to confirm the Iinclings of I"~/~~ITTEXBERC, (1958), but in his experilllent tlie fish had a gas content of 1.2 cc after 40 clays.

Tlie present ~vorlc intends to give inore information about swimbladder function of the fish in tlieir natural environrneiit.

M A T E R I A L

Local lierr~lig (Clupen hniengur) froin the Tronclheirns florcl crncl the Llnd%spoll lravc becn ~lsed. They are niore or lesr available the entire ycar and .i\7erc caught by hook ancl line or by gillnets. For comparative reasons char (Snlnzo n@znus) from Grimevatnet, Vangsvatiiet near Bergen ancl Bvcrvatnet in Salangen have been studied. T h e char Ivere caught by gillnets and beach seine. Tlie reason for using cliar as a colnparative fish in this stucly is iiot only the siliiilar ethology ivith respect to hydrostatic pressure, but also the far better resistalicr of the cliar against experimental stress. Both herring ancl char were transported to the institute of Marille Research for experimental -\\iork.

M E ' I ' H O D S .AND R E L I A B I L I T Y 0 1 7 DATA

T h e s1\7imbladder gas Ivas analysccl usiiig the 112 cc analyzer (SCHO-

LANDER 1947) in the laboratory experiments. I n the field work tlie wvater analyzer)) (SCHOLANDER, VAX DAM, CLAFF ancl KANWISIIER 1955) was usecl. T h e accuracy of the latter is not goocl ellough for CO, analysis, but accuracy of the values of the gas lnixtures founcl in tlie physostoiiies (SUNDNES 1963, S U N D N E ~ , BRATLAND ancl STRAND 1969) are cletcrniined by the accuracy of the analyzcr \\~hicli is ⁱ0.3 per cent of the true value.

Buoyancy tests were perforniccl by weighing the gas free fish under water as describecl by SAUNDERS (1953). T h e voluliietric measurement of the gas c o ~ i t e i ~ t gave the clcptli of buoyancy by using following equa-

Gas volume tioli: Depth of buoyancy =

Weight of the submerged fish

A nieasu~cnicnt ol tlic buoyancy o-t pl~ysostoliies like herring and char by method rnentioliecl above is not precise. By talting the fish from tlie depth of catcli the s\\liliiblaclder cxpaliclr alicl thereby some gas 1s released.

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Buoyancy measurements of char are inore problematic as it releases gas even in a b u ~ y a n t conclition by eil~otiolzal reactions. The buoyancy cletcrmiila-lions ~vill therefore always give m i n i i ~ ~ u n i values. T h e swimbladder gas pressure reiative to tlze surrounding hydrostatic pressure tvas nleasured by transducers (SUXDXET and GY'~IXE 1972, in press).

These measkrrements were carried out in aquaria. 'These results arc also influellced by the ernotions of the fish. I n experiinents it is clifficult to decide ~ v h i c l ~ aspect of the beha\~iour is illlitatecl in thc instinctive system of the fish. T o make tlic fish cooperate in aquaria ancl obtain data wl-rich is relevant to the open sea is, therefore, clifficult in lllallj7 cases. T h e Lish have bee11 ob~ervccl in fielcl by various types of Sinlraci echo sounder.

R E S U L T S A N D D I S C U S S l O K

T h e value of ihe C Q L and 0, contents founcl in the s~vimbladders of herring in the Troildheiins ljord a t 8 m depth uncles a pressure conclition tvhcre they released gas (n~illions of gas bubbles reaching [he surface over the 1\7holc visiblc area) are given in Table 1.

Table 1. Gas content in Clzijea hn~etlgzlr from Tronclheimsijorcl caughl by hook and line.

Fish no.

r 7

b he oxygen content of the swilnhlaclder variecl from 8.5 to 18.3 ~ 0 1 7 6 .

I11 herring fro111 the I,ind?ispnll the oxygen ccntcnt variecl over the same range as in the Tro~~clhcims fjord herring, i.e. 7.7 to 17.7 vol?& (Table 2).

As far as the gas data are concerned, the oxygen values do not incli- cdtc that tlzc srvin~b~acldcr gas origillated from the x~ascular system of the fish. A gas content of > 21 volO/, oxygen tvoulcl eviclcntly inclicate that the oxygen in thc haenloglobin of the vascular system of the fish coulcl play a n i~npol-tant part as the sourcc of the s~vimblaclder gases. T h e field studies have not so far indicateci the possibility that the oxygen originates froin the haernoglobin of the vascular system.

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Table 2

.

Gas content in Clujea harengus from Lind5spollen .

Fish no

. 1

^CO.%

1

^OzOA

Table 3

.

Gas contelit in Salwzo nlpinus from Ratldsfjorden .

Depth in meters

1

^GOz%

1

^0.%

The oxygen content found in the swimbladder of the char is also lower than the atmospheric value

.

I n general. the oxygen values from char in normal conditions in nature are lower than those found in herring

.

I n Table 3. 4 and 5 the 0. content with the respective C 0 2 values from different localities and depths are listed

.

While the values for the gas mixture found in herring and the char do not exclude the possibility that the gas has been ccswallowed>> directly from the atmosphere. they are more consistent with the findings of

JACOBS (1934). SRIVASTAVA (1964) and FAHLEN (1967)

.

O n the other hand WITTENBERG (1958) and FAHLEN have described a refilling of the swim-

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bladder in Salmonids independent of an air swallowing function. They found that the gas content of the swimbladder mainly consisted of nitrogen. The refilling process was very slow and incomplete in the fishes used by FAHLEN (1967) compared to those used by WITTENBERG (1958).

Table 4. Gas content in S a b ~ o alpi~zus f r o m Grimevatnet.

Depth i n Depth i n

meters meters

)

^CO,% ^I

I

^{0 2 %}

I

I ^I

I

I n the latter experiments, the high nitrogen content does not indicate the source of the gas. I t is cloubtful that the primary gas deposited in the swirnbladder consists mainly of nitrogen. As long as the composition of the residual gas in the swiinbladder is unknown, any refilled gas will be cccontaminatedn by the residual gas (SUNDNES et al. 1969). Also, the delay between the deposition of gas and the gas sampling is important (SUNDXES et al. 1969). I n the present work char have been emptied of swimbladder gas to the degree which is possible by puncture and then kept in aquaria with no access to the surface. The fish were then emptied of gas in inter- vals varying from 3 to 9 days. The results of the gas analysis for a char are shown in Table 6.

Table 6 shows a high degree of variation of the oxygen content in the refilled gas. The oxygen content of the primary refilled gas is not known as long as the residual gas volume is unknown.

T h e high nitrogen content found in the refilled gas by WITTENBERG (195%) is possibly caused by the secondary effect described by S U W D N E ~ et al. (1969). The relatively high oxygen content found in the present

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'Table 5 . Gas content in Salnzo (r&ztzz~s from Dvcrvatnet

.

Depth in melels

Date

1

Depth in meters

Table 6

.

Gas content of char with nc excess to surface .

original gas l.efillec1 gas rel'illccl gas refilled gas refilled gas refillecl gas refillecl gas

--

vol . of gas

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cxpcrinients and also the oxygen content founcl in herring by FAIILEX (1957) may indicat:: that the vascular systeiii or tissue surrouiiding the swimbladcler are also a souice ot suvi~iiblaclcler gas.

If the gas in the s\i7imblaclcler is cleposlted there only

by

swallowing or other injecting processes clirect from the gaseous atmosphere, it means that the fish must bc heavily overinflated at the surface in order to be buoyant at, e.g. 100 ¹¹¹clepth. O n the other hand the fish call have a gas volrrme ~vhich is able to keep then1 buoyant in the upper few meters and thereby be in negative buoyancy at any depth below that.

Herring liiay carry out evtcncleci vertical migrations, i.e. over pressure ranges up to 40 atmospheres. Herring in the present invrstigation caught a t 10 in depth hacl a gas volunie of 6 cc. If one assullies that the hcrring

\vas buoyant at that cieptli, it must then take in ailother 458 cc of air a t the surface to be buoyant a t 400 ~n depth. Such a situation of the herring has so far not f~een observed in our invertigations.

The other possibility is to keep the s\vimbladder volume constant by the rigidity of the s\vimblaclcler or the body ~vali. By using a transduser technique (SUNDYES ancl GYTRE 1972, in press), the prcsure of the swimbladder gas in livc herring Tvas measured in relatioil to the surroulzding hydrostatic pressure. The internal gas pressure of the herring follo~ved the hydrostaiic pressure on the outside. Whether minute pressure clifferences arc possible in herring ~vliich are <ccooperating)) during the expcriiiient is unltrto~vn, but it is clearly evident that no physical rigiclity exists in the herring ~vllich can lceep the s~\iirnbladdcr volume constant over those pressure ranges \\/here herring perforin vertical migr'ltions.

This means that only by access to atmospheric gases a t f h c iurfnl ~vill herring not be buoyant in the major clepth area of their ~[~stribution.

Against this opinioii numerous echo sounding observations of gas release indicate an overflotation of the herring from all depths when migration occurs to~varcl clecrcasing hydrostatic pressure. At the surface this is also

R E L E A S E D G A S

---- H E R R I N G S C WOOL

Fig. 1. Echogram sho~ving gas release from ~nigratillg herring. (Photo : G. \icstnes)

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visible by eye as previously described in the present chapter. I n Fig. 1 gas release situation of herring is shown.

As previously discussed, buoyancy measurements of herring and char are problematic, but a few observations indicate that both species are buoyant at their depth of catch (Fig. 2 ) .

0

- SURFACE

DEPTH O F CATCH ( M )

Fig. 2. Depth of buoyancy of Clupen hnrengzls

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ancl Snl~no n@iizus (0) in relation to depth of catch. The diagonal is thc line of perfect buoyancy, i.e. wherc depth of buoyancy cquals clepth of catch.

The findings presented here indicate that both the herring and the char are able to compensate for the volume loss of the swinlbladder due to hydrostatic pressure and without access to the surface.

Another question which is related to this type of swimbladder concerns their compressibility due to the effect of migration towards higher hydrostatic pressure as previously clescribed. There are no indications that the herring compensate for the increased hydrostatic pressure as fast as the vertical migration takes place. As the swimbladder ancl its geometry are known to play an important part of the target strength in acoustical lnethods (MCCARTNEY and STUBBS 1971), further investigations must elucidate an eventual effect on the target strength of Clupeids and Sal- rnonids due to vertical migration and during the restoration of neutral buoyancy at a certain depth.

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97

A C K N O W L E D G E M E N T

The authors thank HANS NORDENG for his help in the field studies in Salangen and Norges alinenviterlskapeligc forskningsrid (N.A.V.F.) and Fiskerinaringens fors0ksfond for financial support.

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Received 10 March 1972 Printed 12 December 1972