The Gemara in Shabbat 26a clearly defines the area in which the hilazon was caught around the time of beginning of the Babylonian exile and probably even before then:
״וּמִדַּלַּת הָאָרֶץ הִשְׁאִיר נְבוּזַרְאֲדָן רַב טַבָּחִים
לְכוֹרְמִים וּלְיוֹגְבִים״. ״כּוֹרְמִים״ — תָּנֵי רַב יוֹסֵף אֵלּוּ
מְלַקְּטֵי אֲפַרְסְמוֹן מֵעֵין גֶּדִי וְעַד רָמְתָא. ״יוֹגְבִים״ —
אֵלּוּ צַיָּידֵי חִלָּזוֹן מִסּוּלָּמוֹת שֶׁל צוּר וְעַד חֵיפָה
Since balsam oil was discussed, the Gemara cites the verse: “But Nebuzaradan the captain of the guard left of the poorest of the land to be vinedressers and husbandmen” (Jeremiah 52:16). The Gemara explains the verse: With regard to vinedressers, Rav Yosef taught: These poorest of the land were the balsam collectors in the south of Eretz Yisrael, in the expanse from Ein Gedi to Ramata. And the husbandmen; these are the trappers of the snail [ḥilazon], from which the sky blue dye is produced in the north of the country, in the area between the Promontory of Tyre and Ḥaifa.
The Zohar, on the other hand, has a very clear assertion that Techelet came from the sea and it explains that this is the Yam Kineret also named in the Zohar as Yam Ginosar.
Zohar 2:48b says
זְבוּלוּן שׁוֹקָא דִּימִינָא דְּגוּפָא הֲוָה, וְיַם כִּנֶּרֶת הֲוָה
בְּעַדְבֵיהּ, וּמֵהָכָא אִשְׁתְּכַח חִלָּזוֹן לִתְכֶלְתָּא
“Zebulun was the market of the right side of the body, and Yam Kineret was in his share and in there it was found the chilazon of the techelet”
Zohar 2:149b says
וּתְכֵלֶת, אָמַר רַבִּי יִצְחָק, תְּכֵלֶת מֵהַהוּא נוּנָא דְּיַמָּא
דְּגִינוֹסָר, דְּאִיהוּ בְּעַדְבֵיהּ דִּזְבוּלוּן. וְאִצְטְרִיךְ
גַּוְונָא דָּא לְעוֹבָדָא דְּמַשְׁכְּנָא לְאִתְחֲזָאָה הַאי גּוָֹון
“And techelet, says Rabbi Yitzchak, techelet is from that fish of the Sea of Ginosar, which is in the portion of Zebulun. And that color is necessary for the avodah of the mishkan to see that color.”
• Birur Shel Peshuto Shel HaTalmud
The Gemara in Megillah 6a based on Devarim 33:19 states that the portion of Zebulun had mountains and seas:
אָמַר זְבוּלוּן לִפְנֵי הַקָּדוֹשׁ בָּרוּךְ הוּא: רִבּוֹנוֹ שֶׁל
עוֹלָם, לְאַחַיי נָתַתָּ לָהֶם שָׂדוֹת וּכְרָמִים, וְלִי נָתַתָּ
הָרִים וְגִבְעוֹת! לְאַחַיי נָתַתָּ לָהֶם אֲרָצוֹת, וְלִי נָתַתָּ
יַמִּים וּנְהָרוֹת! אָמַר לוֹ: כּוּלָּן צְרִיכִין לָךְ עַל יְדֵי
חִלָּזוֹן, שֶׁנֶּאֱמַר: ״[עַמִּים הַר יִקְרָאוּ] וּשְׂפוּנֵי טְמוּנֵי
The verse should be interpreted as follows: Zebulun said before the Holy One, Blessed be He: Master of the Universe! To my brothers, the tribes whose territory is adjacent to mine, You gave fields and vineyards, whereas to me You gave mountains and hills; to my brothers You gave lands, whereas to me You gave seas and rivers. God said back to him: Nevertheless, all will need you due to the ḥilazon, the small sea creature residing in your territory that is the source of the dye used in the ritual fringes [tzitzit]. As it is stated in Moses’ blessing to Zebulun: “They shall call the people to the mountain: There they shall sacrifice offerings of righteousness; for they shall suck of the abundance of the seas, and of the hidden treasures of the sand” (Deuteronomy 33:19).
Josephus in Antiquities of The Jews, Book V, Chapter 1, Par. 22 states :
The tribe of Zebulon's lot included the land which lay as far as the
Lake of Genesareth, and that which belonged to Carmel and the sea.
Since Zebulun was the only tribe that was catching the chilazon, then Zebulun’s share can be simply understood as the area of mount Carmel (and Modern Haifa) and its seashore, the mountains of Tzfad and the west coast of the Sea of Galilee.
This does not mean that other non-Israelite people were not catching the chilazon in other areas and indeed we know that the Phoenicians who were north of Zebulun’s share also caught the chilazon in their Mediterranean waters, but it seems the Jews only did so in the area that corresponded to Zebulun.
The Gemara in Sota 46b is more specific and states that the Techelet preferred by the Jews was not dyed by the seashore but in a city called Luz:
וַיֵּלֶךְ הָאִישׁ אֶרֶץ הַחִתִּים וַיִּבֶן עִיר וַיִּקְרָא שְׁמָהּ
לוּז הוּא שְׁמָהּ עַד הַיּוֹם הַזֶּה תַּנְיָא הִיא לוּז שֶׁצּוֹבְעִין
בָּהּ תְּכֵלֶת הִיא לוּז שֶׁבָּא סַנְחֵרִיב וְלֹא בִּלְבְּלָהּ
נְבוּכַדְנֶצַּר וְלֹא הֶחְרִיבָהּ וְאַף מַלְאַךְ הַמָּוֶת אֵין לוֹ
רְשׁוּת לַעֲבוֹר בָּהּ אֶלָּא זְקֵנִים שֶׁבָּהּ בִּזְמַן שֶׁדַּעְתָּן
קָצָה עֲלֵיהֶן יוֹצְאִין חוּץ לַחוֹמָה וְהֵן מֵתִים
“And the man went to the land of the Hittites, and he built a city, and he called its name Luz; that is its name to this day” (Judges 1:26). It is taught in a baraita: This is the city Luz where sky blue wool is dyed. It is the same city Luz where, although Sennacherib came and exiled many nations from place to place, he did not disarrange and exile its inhabitants; Nebuchadnezzar, who conquered many lands, did not destroy it; and even the angel of death has no permission to pass through it. Rather, its Elders, when they have decided that they have reached the end of life, go outside the city wall and die.
This city was not by the Mediterranean sea but the Ben Yehoyada ad loc. explains that they would bring dying material (either the chilazon alive or its dry glands) from the sea until Luz and dye them there as it would come out nicer.
We see in Sanhedrin 12a how two rabbis from Tiberias, the last location of Sanhedrin, had secured techelet from Luz and intended to send it to Babylon but were prevented from doing so.
There is also evidence of chilazon shells up to the mountains on the way from the Mediterranean Sea to the Sea of Galilee which may be explained by the fact that the whole region was producing murex products and trading them but the techelet from Luz was preferred among the sages.
While we can’t know where Luz was located, it seems that it was in the northern part of Israel (Megillah 6a), around the shore of the Sea of Galilee, and not too far from Tiberias (Sanhedrin 12a).
•Birur Shel Peshuto Shel HaZohar
The Zohar 3:163b mentions that יָ''ם, בְּהַהוּא תְּכֵלֶת דְּנוּנָא, דְּשִׁבְעִין דַּרְגִּין דְּיַמָּא the ‘sea’, in which (there is) the techelet from the fish, from the seventy levels of the sea” and Zohar 2:23a says
שִׁבְעָה עַמּוּדִים, דְּאַרְעָא סְמִיכָא עָלַיְיהוּ. וְאִינּוּן
שִׁבְעָה יַמִּים. וְיַם כִנֶּרֶת שַׁלְטָא עָלַיְיהוּ. אָמַר רִבִּי
יְהוּדָה, לָא תֵּימָא דְּשַׁלְטָא עָלַיְיהוּ, אֶלָּא דְּאִתְמַלְיָיא
In short, according to these Zoharic passages, Yam Kineret rules over the seven seas which is clarified in the passage as meaning that it gets water from all of them and the fish of the techelet is from these 7 seas that are 70 levels.
Hence, these two passages could be understood as implying that the fish from which techelet dye is extracted, the chilazon, can be found not just in the Yam Kineret but at least it is also found in some of the seven seas that flow into the Yam Kineret, according to the Zohar, such as the Mediterranean Sea (See Bava Batra 74b) where the Gemara says the chilazon is found.
On the other hand, since techelet (the final product) was dyed and traded in the area of the Sea of Galilee, the Zohar 2:149b mention of Ginosar may refer to the whole area where techelet came from, the whole region of the Sea of Galilee, (see this expansive meaning also found in Rashi and Steinslatz English Perush on Eruvin 30a גינוסר - ארץ ים כינרת ופירותיה מתוקין).
Furthermore, Zohar 2:48b could be understood as that in the Sea of Galilee the chilazon extract or dry glands ready for dying were ‘found’ to be purchased by techelet artisans.
(As a way of remez, the word אִשְׁתְּכַח in Zohar 2:48b can be rendered in Aramaic as ‘forgotten’ (See Jastrow and the Gemara examples in there). If so, the Zoharic passage would mean that in the Sea of Galilee the identity of the chilazon was finally forgotten as the last Sanhedrin was dismissed in 425 CE after the death of Gamaliel IV.)
In another vein, the Baal Hatechelet suggested that the Zohar was describing the period before the Babylonian exile and the Talmud described the period of the Second Temple and afterward until the chilazon was ultimately lost. This resolution is problematic due to the fact that the Zohar is attributed to Shimon Bar Yochai who lived and potentially described the situation in his time and we have no external or internal records of the chilazon being found exclusively in the Sea of Galilee from the period of the First Temple. Actually, if we were to go down this path, it would appear to be the opposite! It could be suggested that the Zohar describes the situation when the Jewish production of techelet took place after the return from Babylon until the times of the Gemara and the Gemara is referring to where during the First Temple period the catching of the chilazon took place (between Haifa and Tyre) and where the sages would obtain techelet in the Second Temple period and afterward.
Nevertheless, it is clear that the Sea of Galilee does not get water from the Mediterranean Sea or the Dead Sea. It is also physically difficult to understand that an animal that lived in freshwater would move or be moved successfully to a salty body of water, namely, the Mediterranean Sea without a miracle that is not explicitly mentioned in any source.
Hence, these passages about the chilazon shouldn’t be understood literally like many other statements of the Zohar in general which seem to be impossible on a physical level al derech hateva, but are speaking of kabbalistic concepts of the Sephirot as commentaries on the Zohar like the Baal HaSulam and Matok MiDvash understand them.
• Can we follow a strictly literal reading of the Zohar in these passages of the Chilazon and techelet against the Talmud?
The Sefer HaKana (ד״ה ענין יראת המקום) says:
So my son, be careful from now on, not to rule based on Aggados that
are not understood except by the baalei hasod.
Therefore, anyone should be wary to pasken Halacha against the Talmud from the Zohar or Midrashim on any topic including the location of the chilazon.
Furthermore, there is a well-established principle that we do not pasken halacha from Midrashim, even less so against the Talmud. (See Talmud Yerushalmi, Chagigah 1:8, 12; Tosafos Yom Tov, Berachot Perek 5, Mishna 4; Nodah B’Yehuda, Yoreh Deah Siman קסא; Chidusei Rashba Megila 15; the Maharal in Beer HaGola, Beer 7, Perek 15.)
Similarly, regarding ruling halacha from the Zohar, it should be mentioned that while many Zoharic practices and stringencies had made their way into Halacha, see Knesses Gedolah quoted by the Magen Abraham 25:20 & Shulchan Aruch HaRav, Orach Chaim 25:28 on how one should follow the Talmud over Zohar on halachic matters when they contradict one another and that Zoharic practices or stringencies no found in the Talmud can’t be enforced on the general public.