Mr Shaw's investigations were carried on for a series of years, both on the fry as it existed naturally in the river, and on captive broods produced from ova deposited by adult salmon, and conveyed to ingeniously-constructed experimental ponds, in which the excluded young were afterwards nourished till they threw off the livery of the parr, and underwent their final conversion into smolts. When this latter change took place, the migratory instinct became so strong that many of them, after searching in vain to escape from their prison—the little streamlet of the pond being barred by fine wire gratings—threw themselves by a kind of parabolic somerset upon the bank and perished. But, previous to this, he had repeatedly observed and recorded the slowly progressive growth to which we have alluded. The value of the parr, then, and the propriety of a judicious application of our statutory regulations to the preservation of that small, and, as hitherto supposed, insignificant fish, will be obvious without further comment.[16]
Having now exhibited the progress of the salmon fry from the ovum to the smolt, our next step shall be to show the connexion of the latter with the grilse. As no experimental observations regarding the future dimensions of the détenus of the ponds could be regarded as legitimate in relation to the usual increase of the species, (any more than we could judge of the growth of a young English guardsman in the prisons of Verdun,) after the period of their natural migration to the sea, and as Mr Shaw's distance from the salt water—twenty-five miles, we believe, windings included—debarred his carrying on his investigations much further with advantage, he wisely turned his attention to a different, though cognate subject, to which we shall afterwards refer. We are, however, fortunately enabled to proceed with our history of the adolescent salmon by means of another ingenious observer already named, Mr Andrew Young of Invershin.
It had always been the prevailing belief that smolts grew rapidly into grilse, and the latter into salmon. But as soon as we became assured of the gross errors of naturalists, and all other observers, regarding the progress of the fry in fresh water, and how a few weeks had been substituted for a period of a couple of years, it was natural that considerate people should suspect that equal errors might pervade the subsequent history of this important species. It appears, however, that marine influence (in whatever way it works) does indeed exercise a most extraordinary effect upon those migrants from our upland streams, and that the extremely rapid transit of a smolt to a grilse, and of the latter to an adult salmon, is strictly true. Although Mr Young's labours in this department differ from Mr Shaw's, in being rather confirmatory than original, we consider them of great value, as reducing the subject to a systematic form, and impressing it with the force and clearness of the most successful demonstration.
Mr Young's first experiments were commenced as far back as 1836, and were originally undertaken with a view to show whether the salmon of each particular river, after descending to the sea, returned again to their original spawning-beds, or whether, as some supposed, the main body, returning coastwards from their feeding grounds in more distant parts of the ocean, and advancing along our island shores, were merely thrown into, or induced to enter, estuaries and rivers by accidental circumstances; and that the numbers obtained in these latter localities thus depended mainly on wind and weather, or other physical conditions, being suitable to their upward progress at the time of their nearing the mouths of the fresher waters. To settle this point, he caught and marked all the spawned fish which he could obtain in the course of the winter months during their sojourn in the rivers. As soon as he had hauled the fish ashore, he made peculiar marks in their caudal fins by means of a pair of nipping-irons, and immediately threw then back into the water. In the course of the following fishing season great numbers were recaptured on their return from the sea, each in its own river bearing its peculiar mark. "We have also," Mr Young informs us, "another proof of the fact, that the different breeds or races of salmon continue to revisit their native streams. You are aware that the river Shin falls into the Oykel at Invershin, and that the conjoined waters of these rivers, with the Carron and other streams, form the estuary of the Oykel, which flows into the more open sea beyond, or eastwards of the bar, below the Gizzen Brigs. Now, were the salmon which enter the mouth of the estuary at the bar thrown in merely by accident or chance, we should expect to find the fish of all the various rivers which form the estuary of the same average weight; for, if it were a mere matter of chance, then a mixture of small and great would occur indifferently in each of the interior streams. But the reverse of this is the case. The salmon in the Shin will average from seventeen pounds to eighteen pounds in weight, while those of the Oykel scarcely attain an average of half that weight. I am, therefore, quite satisfied, as well by having marked spawned fish descending to the sea, and caught them ascending the same river, and bearing that river's mark, as by a long-continued general observation of the weight, size, and even something of the form, that every river has its own breed, and that breed continues, till captured and killed, to return from year to year into its native stream."
We have heard of a partial exception to this instinctive habit, which, however, essentially confirms the rule. We are informed that a Shin salmon (recognized as such by its shape and size) was, on a certain occasion, captured in the river Conon, a fine stream which flows into the upper portion of the neighbouring Frith of Cromarty. It was marked and returned to the river, and was taken next day in its native stream the Shin, having, on discovering its mistake, descended the Cromarty Frith, skirted the intermediate portion of the outer coast by Tarbet Ness, and ascended the estuary of the Oykel. The distance may be about sixty miles. On the other hand, we are informed by a Sutherland correspondent of a fact of another nature, which bears strongly upon the pertinacity with which these fine fish endeavour to regain their spawning ground. By the side of the river Helmsdale there was once a portion of an old channel forming an angular bend with the actual river. In summer, it was only partially filled by a detached or landlocked pool, but in winter, a more lively communication was renewed by the superabounding waters. This old channel was, however, not only resorted to by salmon as a piece of spawning ground during the colder season of the year, but was sought for again instinctively in summer during their upward migration, when there was no water running through it. The fish being, of course, unable to attain their object, have been seen, after various aerial boundings, to fall, in the course of their exertions, upon the dry gravel bank between the river and the pool of water, where they were picked up by the considerate natives.
No sooner had Mr Young satisfied himself that the produce of a river invariably returned to that river after descending to the sea, than he commenced his operations upon the smolts—taking up the subject where it was unavoidably left off by Mr Shaw[17]. His long-continued superintendence of the Duke of Sutherland's fisheries in the north of Scotland, and his peculiar position as residing almost within a few yards of the noted river Shin, afforded advantages of which he was not slow to make assiduous use. He has now performed numerous and varied experiments, and finds that, notwithstanding the slow growth of parr in fresh water, "such is the influence of the sea as a more enlarged and salubrious sphere of life, that the very smolts which descend into it from the rivers in spring, ascend into the fresh waters in the course of the immediate summer as grilse, varying in size in proportion to the length of their stay in salt water."
For example, in the spring of 1837, Mr Young marked a great quantity of descending smolts, by making a perforation in their caudal fins with a small pair of nipping-irons constructed for the purpose, and in the ensuing months of June and July he recaptured a considerable number on their return to the rivers, all in the condition of grilse, and varying from 3lbs. to 8lbs., "according to the time which had elapsed since their first departure from the fresh water, or, in other words, the length of their sojourn in the sea." In the spring of 1842, he likewise marked a number of descending smolts, by clipping off what is called the adipose fin upon the back. In the course of the ensuing June and July, he caught them returning up the river, bearing his peculiar mark, and agreeing with those of 1837 both in respect to size, and the relation which that size bore to the lapse of time.
The following list from Mr Young's note-book, affords a few examples of the rate of growth:—
List of Smolts marked in the River, and recaptured as Grilse on their first ascent from the Sea.
| Period of marking. | Period of recapture. | Weight when retaken. |
| 1842. April and May. | 1842. June 28. | 4 lb. |
| July 15. | 5 lb | |
| 15. | 5 lb. | |
| 25. | 7 lb.[18] | |
| 25. | 5 lb. | |
| 30. | 3½ lb.[18] |