Stunting and parr-reversion during smoltification of coho salmon (Oncorhynchus kisutch)

91

28 (1982) 91-104 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Aquaculture,

STUNTING AND PARR-REVERSION COHO SALMON (ONCORHYNCHUS

DURING SMOLTIFICATION KISUTCH)

LEROY C. FOLMAR, WALTON W. DICKHOFF*, F. WILLIAM WAKNITZ

OF

CONRAD V.W. MAHNKEN and

Northwest and Alaska Fisheries Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montfake Boulevard East, Seattle, WA 98112 (U.S.A.) *Department

of Zoology,

University

of Washington,

Seattle,

WA 98195

(U.S.A.}

(Accepted 15 January 1982)

ABSTRACT Folmar, L.C., Dickhoff, W.W., Mahnken, C.V.W. and Waknitz, F.W., 1982. Stunting and pair-reversion during smoltification of coho salmon (Oncorhynchus kisutch). Aquaculture, 28: 91-104. Two forms of seawater-adapted coho salmon par-r, the “stunt” and the “par-r-revertant” are described. The “stunt” presumably results from premature transfer to seawater, while the “par-rrevertant” may occur when smolts are transferred to seawater during desmoltification. In general, the condition of seawater-adapted parr may be described as hypofunctional in both development and osmoregulatory ability. Accumulated data regarding the morphology and physiology of these two developmental forms are presented.

INTRODUCTION

The first paper in this symposium (Gorbman et al., 1982) addressed some of the difficulties attendant to the establishment of a suitable nomenclatorial scheme for the various stages of the parr-smolt transformation in coho salmon (Oncorhynchus kisutch). The purpose of this paper is to review available information on recently recognized variations in that transformation, the phenomenon of seawater-adapted parr. In 1978, Bern speculated that seawater-adapted parr were the result of abnormal smoltification. Our more recent observations suggest that there are different biochemical and endocrinological pathways of development for the seawater-adapted smolt and seawater-adapted parr. However, it appears that the seawater-adapted par-r may be a natural component of any year class of coho salmon suggesting, in part, a genetic basis for their existence. The remainder of this report describes the possible sources of seawater-adapted parr and the physiological and morphological characteristics of two forms of the seawater-adapted Parr, the “stunt” and the “par-r-revertant”. 0044-8486/82/0000-0000/$02.75

0 1982 Elsevier Scientific Publishing Company

92 SOURCES OF SEAWATER-ADAPTED

PARR

Seawater (SW)-adapted coho salmon parr arise from the transfer of juvenile fish from fresh water (FW) to SW at an inappropriate time during their development. As an aid to discussion, the possible sources of SWadapted parr are outlined in Fig. 1. In this representation, normal smoltification of coho salmon progresses from fry to parr (A) and then par-r to smolt (B) in FW. The fully smolted fish then enters SW to continue development into the adult phase. As pointed out by Gorbman et al. (1982), smoltification is a continuous process with many subtle changes occurring during this developmental period. If early parr stages (E and F) are prematurely transferred to SW, they retain the characteristic appearance of the parr and have been referred to as “stunts” (Clarke and Nagahama, 1977). Since this is a dynamic process, transfer of progressively larger parr (e.g. F>E) to SW results in progressively larger “stunts”. The usual fate of the “stunt” is death; however, some may survive and smelt in SW the following spring. When junveile coho salmon reach the stage of a true smolt in fresh water, they are ready to enter SW and mature to adulthood (C). However, if true smolts are prevented access to SW, or cannot maintain a requisite critical size to enter SW after summer solstice (Kubo, 1974; Mahnken and Waknitz, 1979; Mahnken et al., 1982), they undergo desmoltification (D) or a regression to a par-r-like appearance. Although these fish regain the external pigmentation of the Parr, they retain many of the morphological characteristics associated with a smolt (Gorbman et al., 1982). ;------___

t

D _____________\

Desmaltlflcatlon

FRY

A

PARR

I I

Smolttftcatlon

B

+

I SMOLT

FW

LSW

SW

SWADAPTED b

E

I

Fig. 1. A diagramatic representation of smoltification sources of “stunts” and “Parr-revertants”.

I

--L

/

MATURING SUBADULT

K

PARR-REVERTANT

in coho salmon showing possible

The “par-r-revertant” (term introduced by Mahnken, 1973) results either from fish entering SW prior to the completion of smoltification (B,I) or from apparent smolts entering SW after the summer solstice but not maintaining a requisite critical size (J) (Mahnken et al., 1982). There are several possible fates for the “par-r-revertant”. The fish may return to FW. Under natural

93

conditions, both masu (Kubo, 1974) and coho salmon (B. Allee, personal communication, 1981) have been observed to enter SW and shortly thereafter return to FW. This entry to SW may be volitional, but in some streams, high flow rates and associated bottom scouring may force the fish into SW for short periods. After returning to FW, these fish will then continue in the FW pattern of desmoltification and likely resmolt the following year. In netpen culture, “par-r-revertants” can be separated from the fully smolted fish and returned to FW facilities. In either case, the larger fish are likely to resmolt as age II+ fish (K), whereas the smaller fish will likely experience osmoregulatory difficulties and die. Although there were no reported observations of coho salmon parr in SW prior to the advent of net-pen culture (Mahnken et al., 1970), we believe that these fish do occur naturally and are not solely the result of the intensive culture practices associated with net-pen culture. What are the similarities and differences between the two forms of SW par-r? Both groups regain the characteristic parr-marks along the lateral line. The “Parr-revertant” (140-180 mm) tends to be larger than the “stunts” (80-130 mm). The condition index (K = 100 X W/L3(g/cm3)) of the “parrrevertant” is generally more smolt-like (0.95-l.O), while the index for the “stunt” more closely resembles that of the FW parr (l.l-1.2), because the “stunt” has not yet experienced the decrease in condition index associated with the smoltification process. Table I lists some of the morphological and physiological differences between “stunts” or “Parr-revertants” and normally developed smolts. We are of the opinion that some of the morphological and physiological characteristics associated with the “stunt” or “parrrevertant” are similar; however, we have yet to make a direct comparison between the two groups in the same population of fish, STUNTS

As shown in Fig. 1, the fish which have been termed “stunt” arise only from premature transfer to SW. There may be some differences in the relative size of the stunts related to the time of SW transfer (i.e., F>E, Fig. 1). Most of the “stunts” tend to remain in the parr state and eventually die. Results from our net-pen studies suggest that some of the prematurely transferred fish appear to go through a stage of “pseudosmoltification” where they appear smolted, but will later be reclassified as “Parr-revertants”. Much of the information available on the physiology and morphology of “stunts” is listed in Table I. This table shows considerable evidence for panhypoendocrine status in both glandular tissue (pituitary, thyroid, inter-renal, pancreas, corpuscles of Stannius and caudal neurosecretory system) and in some peripheral tissue (kidney and liver; growth hormone and triiodothyronine receptors). Although a large amount of information has been accumulated (Clarke and Nagahama, 1977; Bern, 1978; Nishioka et al., 1982), it is still not clear which endocrine deficiency, if any, is the primary cause of “stunting”.

94 TABLE I Some morphological and physiological characteristics of “stunts”, normally developed smolts ___.__.~__.._.~ .-...

“Parr-revertants”, and

Tissue

Description

Reference

Pituitary

General appearance of the pituitary was smaller and flatter in SW stunts than in normal SW smolts.

Clarke and Nagahama (1977)

No distinct difference.

Bern (1978)

SW stunt had a greater number of corticotropes than did normal SW smolt . “Haloed” secretory granules of SW fish showed features of high synthetic activity, including well-developed rough endoplasmic reticula (RER) and Golgi complexes. They also had large intercellular spaces between corticotropes adjacent to neurophysis. When SW stunts were returned to FW, these cells were indistinguishable from normal FW Parr.

Clarke and Nagahama (1977)

Appear inactive in both normal smolts and stunts in SW.

Bern (1978)

rostra1 pars distalis (RPD) corticotropes

mammotropes

Ventral RPD region considerably smaller in Clarke and Nagahama SW stunts compared to SW smolts due to (1977) the atrophy of mammotropes. In stunts, nuclei are heavily invaginated with abundant chromatin at periphery. In SW stunts the cytoplasm was sparse with scattered ER, reduced Golgi complexes and widened intercellular spaces. These cells also showed occasional vacuolization. When stunts were returned to FW, RPD became considerably enlarged due to hypertrophy of mammotropes. In FW the dense chromatin was no longer observed at the periphery of the nucleus. The returned stunts also had a single prominent nucleolus, the ER was hypertrophied, large mitochondria were abundant, and the Golgi complexes were enlarged and had numerous secretory granules associated with them. proximal pars distalis (PPD) thyrotropes

Appeared less active in SW stunts than in SW smelts.

Bern (1978)

Showed features of reduced synthetic activity in SW stunts.

Clarke and Nagahama (1977)

95

Tissue

Description

Reference

gonadotropes

Anneared to be fewer cells in SW stunt than Clarke and Nagahama normal SW smolt . (1977)

somatotropes

Highly active in SW smelts with dilated RER, many Golgi complexes and developing granules. These cells were also active in SW stunt but were reduced from the activity observed in SW smolts.

Bern (1978

The PPD was the most prominent lobe of the pituitary in SW stunts. Somatotropes comnrised much larger nortion of the cords of the PPD in stunts than in normal smolts in SW. Somatotropes were more active in the peripheral cords than in the interior. In the SW stunts these cells contained numerous secretory granules, a well-developed RER and prominent Golgi complexes. There were also abundant mitochondria in the vicinity of the Golgi complexes. When the stunts were returned to FW the number of secretory granules were reduced in the somatotropes.

Clarke and rTagahama (1977)

In most stunts, neurohypophysial area closest to somatotropes was strikingly degenerated although the basal lamina remained intact. When stunts were returned to FW, the neurohypophysial region adjacent to the somatotropes regenerated.

Clarke and Nagahama (1977)

Thyroid cells significantly shorter in SW stunts than SW smolts. Cells appeared less active in the SW stunt with reduced ER and Golgi complexes and few vescicles. Appeared hypothyroid.

Bern (1978)

Thyroid cells were significantly shorter in SW stunts than SW smolts. Nuclei were smaller and more indented in SW stunt. There were also fewer resorptive vacuoles in stunts.

Clarke and Nagahama (1977)

No significant differences in major nuclear diameters between SW stunts and SW smolts. The SW smelts had larger interrenal nuclei than FW Parr. The SW stunt had abnormally formed mitochondria with small cristae. Interrenal cell membrane was considerably convoluted adjacent to endothelial basement membrane in SW smolts, but in SW stunts the cell membrane was more smooth in outline (electron microscopy ).

Bern (1978)

No obvious differences (light microscopy).

Clarke and Nagahama (1977)

I

neurohypophysis

Thyroid

Interrenal

_

96

Tissue

Description

Reference

Pancreas

Islet cells smaller in SW stunts than in SW smolts.

Bern (1978)

All endocrine cell types were smaller with many highly indented nuclei in the SW stunts.

Clarke and Nagahama (1977)

Cells were smaller in SW stunt than in SW smolt .

Bern (1978)

Two cell types (Type 1, large granules; Type 2, small granules) increased in size during normal smoltification. Mean cell size of SW stunt similar to FW Parr. In SW stunts cells appeared active and relatively depleted of granules.

Aida et al. (1980)

Caudal neurosecretory system

SW stunt tended to have less cellular cyto. plasm, active organelles and granules in neurons than did SW smolts. Urotensin content significantly lower in SW stunt than in SW smolt.

Bern (1978)

Gill

Gill membrane preparation in SW stunt showed reduced capacity to bind tilapia growth hormone (tGH) when compared with SW smolt.

Fryer and Bern (1979)

Gill Na’-K’-ATPase activity significantly greater in SW Parr-revertant than in normal SW smolt .

Folmar and Dickhoff (this paper)

Skin

In epidermis taken from ventrolateral surface, SW stunts had fewer mucous cells and the skin was only half as thick as in normal SW smolts.

Clarke and Nagahama (1977)

Scales

Both the size and number of scale circuli in SW stunts were less than in normal SW smolt .

Gorbman et al. (1982)

Muscle

SW stunts had much higher muscle protein concentrations than did SW smolts. There was no difference between SW stunts and smolts in the amounts of tissue water.

Woo et al. (1978)

Liver

Liver membrane preparations from SW stunts showed lower binding affinity for tGH than a similar preparation from SW smolts.

Fryer and Bern (1979)

Liver nuclei from SW stunts had lower thyroid hormone binding efficiency than did a similar preparation from SW smolts.

D.S. Darling and W.W. Dickhoff (unpublished observations, 1981)

Corpuscles of Stannius

97 Tissue

Description

Reference

Kidney

Kidney membrane preparations from SW stunts showed lower binding affinities for tGH than did similar preparations from SW smolt .

Fryer and Bern (1979)

Posterior intestine

Net water flux accross the posterior intestine was 150 greater in SW stunt than in normal SW smolt.

Collie and Bern (1980)

Accessory appendages

Paracloacal skin folds and auxiliary appendages of the pelvic fins of SW parrrevertants appeared to be more similar to SW smolts than FW pan.

Gorbman et al. (1982)

Dentition

The teeth of the jaws and tongue of SW parr-revertants were well-developed compared to FW pan, although they were not as advanced as those found in a normal SW smolt .

Gorbman et al. (1982)

Plasma sodium (Na’)

No difference in plasma Na’ concentrations between SW stunts and normal SW smolts.

Clarke and Nagahama (1977)

glucose

Stunts did not develop hypoglycemia after SW entry, normal smolts did.

Woo et al. (1978)

insulin

Insulin levels were lower in SW stunt than SW smolt.

Bern (1978)

Plasma concentrations of T, were lower in Parr-revertant than SW smolts.

Dickhoff and Folmar (this paper)

No difference in plasma concentrations of T, between SW Parr-revertants and SW smelts.

Dickhoff and Folmar (this paper)

thyroxine

(T, )

triiodothyronine (T,)

The time-course changes in plasma levels of thyroxine (T,) during the period of development from the parr to the smolt stages in FW (Dickhoff et al., 1978) are presented in Fig. 2 (solid line). The onset of increased plasma (T4) levels appears related to the geographic locations of the fish, whereas, the peak plasma levels of T, appear to be related to the phase of the moon (Grau et al., 1981). The stage of development labelled as E in Fig. 1 corresponds to the are labelled as 1 on the T4 curve (Fig. 2). Those fish labeled F in Fig. 1 correspond with the area between points 1 and 2, respectively, in Fig. 2. It has been previously shown (Folmar and Dickhoff, 1981; Dickhoff et al., 1982) that the greatest success in smoltification of coho salmon results when developing fish enter SW at approximately the area labelled 4 on the curve (approximately 65-85s complete). When developing fish are directly transferred to SW at the points corresponding to 2 or 3 on the T, curve, there is a rapid decrease in the circulating levels of T, (dashed lines). This suggests that premature transfer to SW reduces thyroid activity and thereby

98

Fig. 2. A representation of a typical surge in plasma thyroxine concentrations smoltification of coho salmon during the spring of the year.

during

limits the period during which successful SW adaptation can occur. This observation agrees with histological observations of thyroids from normal and “stunted” fish (Clarke and Nagahama, 1977; Bern, 1978). Premature transfer to SW also results in the cessation of growth which could be related to changes in metabolism mediated by thyroxine (cf. Folmar and Dickhoff, 1980) or also to the influence of thyroxine on the production of growth hormone (GH) in the somatotrophs (Higgs et al., 1976) and possibly the priming of GH receptor sites in the peripheral tissue (Fryer and Bern, 1979). In cases where fish are transferred directly from FW to SW (net-pen culture or ocean ranching), we have suggested that transfer to SW near point 4 (Fig. 2) will result in the greatest survival of those fish (Folmar and Dickhoff, 1981). However, recent evidence suggests that fish released from Columbia River hatcheries near points 2 or 3 in the developmental sequence may continue to develop the riverine (FW) phase of migration, so that they are at or near point 4 by the time they enter SW (Zaugg, 1982). PARR-REVERTANTS

Fish in this category may result from entry into SW prior to tion of smoltification (G, Fig. 1) or during the desmoltification Fig. 1). In relation to the previously described T, cycle in FW, revertant” can enter SW near points 2-3 in Fig. 2 (corresponds

the complephase (D, the “parrto G, Fig. 1)

99

or afterthe period we consider optimal for successful transition into SW (5, Fig. 2). In FW, the period of desmoltification (D, Fig. 1; after point 5, Fig. 2), occurs after summer solstice, on the declining photoperiod. Those fish entering SW prematurely (G, Fig. 1; prior to point 3, Fig. 2) or during desmoltification (D, Fig. 1; after point 5, Fig. 2) which cannot maintain the required growth in SW will gradually regain some of the pigmentary characteristics of parr (Prentice et al., 1981), but they will also have some morphological characteristics of smolts (Gorbman et al., 1982). The “Parr-revertant” may die or resmolt the following spring (as age II animals). Death probably results from osmoregulatory failure. For the survivors, growth and development appear to be compromised to maintain osmoregulation. We have found that “par-r-revertants” have significantly higher gill Na+-K+-ATPase activity (Table II) than normally developed smolts. Also, water transport in the hind gut appears to be compromised in the “Parrrevertant” (Collie and Bern, 1980). These fish have also been reported to contain higher plasma chloride ion concentrations than normal smolts (Kubo, 1974). TABLE II Concentration of plasma thyroid hormones and gill Na+-K+-ATPase of ye_arling coho salmon smolt and Parr-revertants after 6 months in seawater. Values are Xk SEM --_._.____ ___.

Smelt Parr-revertant

Thyroxine

Triidothyronine

Gill Na+-K+-ATPase

11.4 + 1.1 6.5 * 0.3

8.1 r 0.1 7.1 + 0.7

18.6 i 0.9 23.7 + 0.9

The T, concentration in the plasma of SW-adapted par-r were lower than those of SW-adapted smolt, while plasma levels of T3 were not significantly different (Table II). Assuming that T4 is the primary secretion from the salmon thyroid (Higgs and Eales, 1977; Milne and Leatherland, 1978), our observations suggest that the Tq secretion rate is depressed in the SW-adapted Parr. This contention is also supported by histological evidence (Table I). Circulating levels of T3 are generally lower than T4 (Folmar and Dickhoff, 1981), but T, appears to be the most potent thyroid hormone in coho salmon (Darling et al., 1980). Although we observed no significant differences in plasma T4 concentration between the SW-adapted parr and smelt, hepatic thyroid hormone receptor-binding efficiency may be reduced in the SW-adapted parr (D.S. Darling and W.W. Dickhoff, unpublished data, 1981). Therefore, thyroid hormone-sensitive peripheral tissue may be considerably less responsive in the SW-adapted par-r. Resmoltification has been observed for “Parr-revertants” overwintering in SW. Fig. 3 shows changes in the percentage of SW-adapted parr in a population of under-yearling (age 0) coho salmon that entered SW in four increments from 9 May through 10 July, 1973. The number of parr in the com-

.-

Mav13.1974

c

March

I= 142mm

11.1974

x- 131 mm

37% Parr

t-

6r

60

P

4% Re-molt

July 30,

80

100

120

140

Fork length

160

180

200

220

(mm)

Fig. 3. Histograms represent the percentages of the size of “psrr-revertants” within a population of subyearling coho salmon transferred to SW from 9 May to 10 July 1973, The percentage of “Parr-revertants” as well as the size of fish reverting to parr increased with time through January 1974. In March, some of the larger “Parr-revertants” had resmolted. By May, over 50% of the parr present in January had resmolted. Arrows denote the mean size of the “Parrrevertants” within the population.

101

posite population increased from 10% in July 1973 to 42% by January 1974. The histograms illustrate that Parr-reversion occurs throughout late summer and into early fall (during the declining photoperiod) and that the size of the revertants increased with time up through November as larger, apparently smolted animals reverted to the parr form (Mahnken et al., 1982). Thus, “Parr-reversion” can occur in populations of coho entering SW long before summer solstice, as well as those entering SW after that time. From November to January there were no significant changes in the size of the fish; however, the number of parr identified in the population increased from 26 to 42%. In January, those fish visually identified as parr were sequestered into net-pens separate from fish visually identified as smolts. As early as March, some of the seawater-adapted parr had resmolted (4% of total population, 10% of the sequestered group) and by May more than one-half of the segregated SW-adapted parr had resmolted (22% of the population). As illustrated by the histogram for May, it was the larger members of the population which resmolted.

IOOl

_November

I

,g)ecember ; January

+

I

’

February 1975

I

I

March

April 4

Fig. 4. Growth of two groups of subyearling, SW adapted, “Parr-revertant” coho salmon overwintering in SW or returned to FW. Lengths are x T SEM. Numbers in parentheses are the percentage of fish which resmolted as yearlings.

102

Another experiment was conducted to determine growth patterns of yearling SW-adapted coho salmon parr which overwintered in SW or were transferred back to FW until the following spring. The two groups were maintained in adjacent circular tanks and fed to satiation with a moist pellet ration. Fig. 4 clearly illustrated that both growth and the proportion of fish resmolting the following spring (numbers in parentheses) were significantly greater in the fish returned to FW than in those fish which remained in SW. The number of resmolted fish in SW for 19’74 (4%, Fig. 3) and 1975 (7% Fig. 4) were similar. Our results were similar to those reported by Clarke and Nagahama (1977). The appetite of our SW-adapted par-r increased upon return to FW, with condition coefficients comparable with parr in FW. In another study (Fig. 5), yearling Parr-revertant coho salmon were transferred from SW to FW in January for a period of 30 days. On 1 February, the fish were transferred back to SW. Upon transfer to FW, plasma T4 levels increased, but when returned to SW 30 days later, the T4 levels returned to those observed prior to FW transfer. The plasma T4 concentrations showed the typical patterns observed for those fish entering SW for the first time (Folmar and Dickhoff, 1981). Gill Na+-K+-ATPase activities dropped upon FW transfer; however, the SW reacclimation patterns were the same as previously reported (Folmar and Dickhoff, 1979).

T

SW (6 month)

FW (30 day)

0

Smelt

A

Parr-revertant

1

3

Days in SW

Fig. 5. Plasma thyroxine concentrations of normal coho salmon smolt and Parr-revertants after 6 months of SW residence (November 1977, Age I), after return to FW for 30 days (January 1978, Age II), and at 1 and 3 days after return to SW (February 1978, Age II).

103

Figures l-5 have shown a variety of possible developmental schemes for coho salmon ages O-II. In Alaska, coho salmon may not smolt until age III or IV. These observations suggest that this developmental versatility in coho salmon may have a genetic basis, which manifests itself through a heterogeneity of developmental rates.

REFERENCES Aida, K., Nishioka, R.S. and Bern, H.A., 1980. Changes in the corpuscles of Stannius of coho salmon (Oncorhynchus kisutch) during smoltification and seawater adaptation. Gen. Comp. Endocrinol., 41: 296-304. Bern, H.A., 1978. Endocrinological studies on normal and abnormal salmon smoltification. In: P.J. Gaillard and H.H. Boer (Editors), Comparative Endocrinology. Elsevier, Amsterdam, pp. 97-100. Clarke, W.C. and Nagahama, Y., 1977. Effect of premature transfer to sea water on juvenile coho salmon (Oncorhynchus kisutch). Can. J. Zool., 55: 1620-1630. Collie, N.L. and Bern, H.A., 1980. Variations in water transport across the coho salmon posterior intestine during smoltification. Am. Zool., 20(4): 873. Darling, D.S., Dickhoff, W.W. and Gorbman, A., 1980. Nuclear thyroid receptors in coho salmon. Am. Zool., 20: 858. Dickhoff, W.W., Folmar, L.C. and Gorbman A., 1978. Changes in plasma throxine during smoltification of coho salmon, Oncorhynchus kisutch. Gen. Comp. Endocrinol., 36: 229-232. Dickhoff, W.W., Folmar, L.C., Mighell, J.L. and Mahnken, C.V.W., 1982. Plasma thyroid hormones during smoltification of yearling and underyearling coho salmon and yearling chinook salmon and steelhead trout. Aquaculture, 28: 39-48. Donaldson, E.M., Fagerlund, U.H.M., Higgs, D.A. and McBride, J.R., 1980. Hormonal enhancement of growth. In: W.S. Hoar, D.J. Randall and J.R. Brett (Editors), Fish Physiology, Vol. 8. Academic Press, New York, pp. 455-597. Folmar, L.C. and Dickhoff, W.W., 1979. Plasma thyroxine and gill Na+-K’ ATPase changes during seawater acclimation of coho salmon (Oncorhynchus kisutch). Comp. Biochem. Physiol., 63A: 329-332. Folmar, L.C. and Dickhoff, W.W., 1980. The parr-smolt transformation (smoltification) and seawater adaptation in salmonids. A review of selected literature. Aquaculture, 21: l-37. Folmar, L.C. and Dickhoff, W.W., 1981. Evaluation of some physiological parameters as predictive indices of smoltification. Aquaculture, 23: 309-324. Fryer, J.N. and Bern, H.A., 1979. Growth hormone binding in tissues of normal and stunted juvenile coho salmon, Oncorhynchus kisutch. J. Fish. Biol., 15: 527-533. Gorbman, A., Dickhoff, W.W., Mighell, J.L., Prentice, E.F. and Waknitz, F.W., 1982. Morphological indices of developmental progress in the parr-smolt coho salmon, Oncor hynchus kisutch. Aquaculture, 28: l-19. Grau, E.G., Dickhoff, W.W., Nishioka, R.S., Bern, H.A. and Folmar, L.C., 1981. Lunar phasing of the thyroxine surge preparatory to seaward migration of salmonid fishes. Science, 211: 607-609. Higgs, D.A. and Eales, J.G., 1977. Influence of food deprivation on radiothyronine and radioiodide kinetics in yearling brook trout (Saluelinus fontinalis (Mitchill) with a consideration of the extent of L-thyroxine conversion to 3,5,3’-triiodo-L-thyronine. Gen. Comp. Endocrinol., 32: 29-40. Higgs, D.A., Donaldson, E.M., Dye, H.M. and McBride, J.R., 1976. Influence of bovine growth hormone and L-thyroxine on growth, muscle composition and histological

104 structure of gonads, thyroid, pancreas and pituitary of coho salmon (Oncorhynchus kisutch). J. Fish. Res. Board Can., 33: 1585-1603. Kubo, T., 1974. Notes on the phase differentiation and smolt transformation of juvenile masu salmon (Oncorhynchus masou). Sci. Rep. Hokkaido Salmon Hatchery No. 28, 17 pp. (in Japanese). Kubo, T., 1980. Studies on the life history of the “masu” salmon (Oncorhynchus masou) in Hokkaido. Sci. Rep. Hokkaido Salmon Hatchery No. 34,95 pp. (in Japanese). Mahnken, C.V.W., 1973. The size of coho salmon and time of entry into seawater: Part I. Effects on growth and condition index. Proc. 24th Annu. NW Fish Cult. Conf., pp. 30-31. Mabnken, C.V.W. and Waknitz, F.W., 1979. Factors affecting growth and survival of coho salmon (Oncorhynchus kisutch) and chinook salmon (0. tshowytscha) in saltwater netpens in Puget Sound. Proc. World Maricult. Sot., 10: 280-305. Mahnken, C.V.W., Novotny, A.J. and Joyner T., 1970. Salmon mariculture potential assessed. Am. Fish Farm., 2(l): 12-15, 27. Mahnken, C.V.W., Prentice, E.F., Waknitz, F.W., Monan, G., Sims, C. and Williams, J., 1982. The application of recent smoltification research to public hatchery releases; an assessment of size/time requirements for Columbia River hatchery coho salmon (Oncorhynchus kisutch). Aquaculture, 28: 251-268. Mime, R.S. and Leatherland, J.F., 1978. Effect of ovine TSH, thiourea, ovine prolactin and bovine growth hormone on plasma thyroxine and triiodothyronine levels in rainbow trout, Salmo gairdneri. J. Comp. Physiol., 124: 105-110. Nishioka, R.S., Bern, H.A., Lai, K.V., Nagahama, Y. and Grau, E.G., 1982. Changes in the endocrine organs of coho salmon during normal and abnormal smoltification an electron-microscope study. Aquaculture, 28: 21-38. Prentice, E.P., Waknitz, F.W. and Mighell, J.L., 1981. Biochemical, morphological and pictorial documentation of smoltification. Appendix E in Assessment of Smoltification and Fitness for Ocean Survival (Quality) of Chinook and Coho Salmon and Steelhead in Columbia River and Puget Sound Hatcheries, Part I. Report for FY 1980-81. A project report to the Pacific Northwest Regional Commission, in press. Woo, N.Y .S., Bern, H.A. and Nishioka, R.S., 1978. Changes in body composition associated with smoltification and premature transfer to seawater in coho salmon (Oncorhynchus kisutch) and king salmon (0. tshazuytscha). J. Fish. Biol., 13: 421-428, Zaugg, W.S., 1982. Relationships between smolt indices and migration in controlled and natural environments. In: Proc. Salmon and Trout Migration Behavior Symposium, June 3-5,198l. University of Washington, Seattle, WA, in press.