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Aquatic creatures: Saltwater vs. Freshwater list, what if one enters the other?
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<blockquote data-quote="77IM" data-source="post: 7586713" data-attributes="member: 12377"><p>A freshwater fish put in saltwater should suffer dehydration as the water leaves its body; a saltwater fish in freshwater will absorb water until it explodes. (Well, its cells explode, which is not as spectacular.)</p><p></p><p>According to teh interwebz, it is a matter of <em>minutes</em> before the fish dies. I think the closest analog in the 5E core rules are the rules for "holding your breath," which gives you 1 + Con mod minutes (min 30 seconds). In theory bigger creatures should have a higher Con but you could do some sort of size modifier if you want to get fancy (a bigger creature should last longer as its surface-area-to-volume ratio is lower).</p><p></p><p>...</p><p></p><p>...</p><p></p><p>...</p><p></p><p>OK, I just fished my copy of <em>Stormwrack</em> out of the library. The rule in question is a bottom-bar on one of the first few pages: When a creature is in the wrong kind of water, it must make a Con check every hour (DC 10, +1 per previous check), or take 1d6 nonlethal damage, and become fatigued until it gets rid of the nonlethal damage.</p><p></p><p>5E doesn't have nonlethal damage, and tends to use exhaustion levels to represent environmental hazards, so I'd adapt this rule as follows: When a creature is in the wrong kind of water, it must make a Con check every hour (DC 10), or take a level of exhaustion that it can't be rid of until it is back in the correct type of water. Note that the <em>very first</em> level of exhaustion will give disadvantage on Con checks, so I don't think the increasing DC is important. Plus only the very largest and toughest of creatures will have a +10 Con mod, and I'm OK with them surviving in the wrong water.</p><p></p><p>...</p><p></p><p>Now, this is all based on real-world fish biology. It wouldn't apply to reptilians, and might not apply to aberrations like aboleths (who knows with those guys), and I'm not sure about arthropods. And merfolk and sea elves are their own thing, so maybe?</p><p></p><p>I feel that worrying about this might hinder game play a bit. However, hand-waving it completely might be unsatisfactory ("Why is there a <em>shark</em> in the lake?" or "Why is there a frog in the ocean?"). One potential solution is that there are certain fish that can exist in both fresh and salt water, salmon being the most famous example. So you could say that merfolk, merrows, sahaguin, etc. are this type of fish. In fact, I'd say any aquatic creature that doesn't clearly belong in one type of water can survive in both.</p><p></p><p>Based on cursory web research, it appears that you can't just dump the fish from one kind of water into the other; in nature the salinity adjusts gradually, and so the fish can't adjust its osmosis thingies right away. So for these fish, I'd adjust the rule so that they still need to make Con checks or get exhaustion, but as soon as they pass a Con check, they've adjusted and the exhaustion goes away at a rate of one level per hour.</p></blockquote><p></p>
[QUOTE="77IM, post: 7586713, member: 12377"] A freshwater fish put in saltwater should suffer dehydration as the water leaves its body; a saltwater fish in freshwater will absorb water until it explodes. (Well, its cells explode, which is not as spectacular.) According to teh interwebz, it is a matter of [I]minutes[/I] before the fish dies. I think the closest analog in the 5E core rules are the rules for "holding your breath," which gives you 1 + Con mod minutes (min 30 seconds). In theory bigger creatures should have a higher Con but you could do some sort of size modifier if you want to get fancy (a bigger creature should last longer as its surface-area-to-volume ratio is lower). ... ... ... OK, I just fished my copy of [I]Stormwrack[/I] out of the library. The rule in question is a bottom-bar on one of the first few pages: When a creature is in the wrong kind of water, it must make a Con check every hour (DC 10, +1 per previous check), or take 1d6 nonlethal damage, and become fatigued until it gets rid of the nonlethal damage. 5E doesn't have nonlethal damage, and tends to use exhaustion levels to represent environmental hazards, so I'd adapt this rule as follows: When a creature is in the wrong kind of water, it must make a Con check every hour (DC 10), or take a level of exhaustion that it can't be rid of until it is back in the correct type of water. Note that the [I]very first[/I] level of exhaustion will give disadvantage on Con checks, so I don't think the increasing DC is important. Plus only the very largest and toughest of creatures will have a +10 Con mod, and I'm OK with them surviving in the wrong water. ... Now, this is all based on real-world fish biology. It wouldn't apply to reptilians, and might not apply to aberrations like aboleths (who knows with those guys), and I'm not sure about arthropods. And merfolk and sea elves are their own thing, so maybe? I feel that worrying about this might hinder game play a bit. However, hand-waving it completely might be unsatisfactory ("Why is there a [I]shark[/I] in the lake?" or "Why is there a frog in the ocean?"). One potential solution is that there are certain fish that can exist in both fresh and salt water, salmon being the most famous example. So you could say that merfolk, merrows, sahaguin, etc. are this type of fish. In fact, I'd say any aquatic creature that doesn't clearly belong in one type of water can survive in both. Based on cursory web research, it appears that you can't just dump the fish from one kind of water into the other; in nature the salinity adjusts gradually, and so the fish can't adjust its osmosis thingies right away. So for these fish, I'd adjust the rule so that they still need to make Con checks or get exhaustion, but as soon as they pass a Con check, they've adjusted and the exhaustion goes away at a rate of one level per hour. [/QUOTE]
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