I read that back in the early days of the Catholic Church that they were heavily invested in the "fishing market." With the popularity of beef on the rise and the loss of revenue to the Catholic Church, they demanded that meat be given up for specific holidays thus increasing the consumption of fish which brought back their revenue stream. Any truth to this?
The ancient fasting practices ruled out not only meat, but fish and many other foods as well, depending on the occasion. In some cases, they ruled out any food whatsoever.
The Didache, a 1st century Christian document, shows that the earliest Christians fasted on Wednesdays and Fridays, though it does not describe exactly what practices were followed. It probably meant complete abstention from food.
The most detailed history of Christian fasting in antiquity comes, I think, from the Church historian Socrates of Constantinople (c 380-439). He is the first to describe, I think, the practice of abstaining from specific foods during Christian fasts. Describing the various Lenten fasts practiced throughout the Church (e.g. three weeks in Rome, six weeks in Greece and Alexandria), he describes the various practices:
One can see also a disagreement about the manner of abstinence from food, as well as about the number of days. Some wholly abstain from things that have life: others feed on fish only of all living creatures: many together with fish, eat fowl also, saying that according to Moses, these were likewise made out of the waters. Some abstain from eggs, and all kinds of fruits: others partake of dry bread only; still others eat not even this: while others having fasted till the ninth hour, afterwards take any sort of food without distinction. And among various nations there are other usages, for which innumerable reasons are assigned. Since however no one can produce a written command as an authority, it is evident that the apostles left each one to his own free will in the matter, to the end that each might perform what is good not by constraint or necessity. Such is the difference in the churches on the subject of fasts. (Ecclesiastical History, V.XXII).
Your question regarded "in the early days of the Catholic Church". I don't know if you had the pre-Schism period in mind or not, but the above provides a synopsis up to the 5th century. In any case, it doesn't seem that the fasting practices within the Church arose because of any heavy investment by the Roman Church. The western Roman Empire was in the process of collapsing during the latter times described by Socrates.
The See of Rome would not acquire any sort of political or commercial power until after the ascent of Charlemagne in the 9th century. Even then, I think it is ingenuous to propose that Roman Christians fasted due to commercial interests of their See. Fasting in the east and the west was rooted in Apostolic tradition, nothing else.
Excursus on 1 Timothy 4:3
1 Timothy 4:1ff is often held up as a sort of condemnation of the Roman Catholic Church (in fact, it would also serve to condemn the Eastern Orthodox Church as well, but those doing the condemning usually have their hands full enough just attacking the Catholic Church and/or are only vaguely familiar with the Orthodox Church). The passage is translated in the King James Bible (i.e. the same King James Bible that originally included the Deuterocanon) as:
Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; Speaking lies in hypocrisy; having their conscience seared with a hot iron; Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.
It is curious that this particular passage regarding commanding to abstain from meats is taken quite literally, yet a passage from the preceding chapter that declares the Church to be the pillar and foundation of truth tends to go unnoticed (1 Timothy 3:15).
There are two points with regard to 1 Timothy 4:3 that are worth noting.
First, the Greek phrase is ἀπέχεσθαι βρωμάτων in the Textus Receptus, the Nestle-Aland Critical Text, and the Patriarchal Text of the Eastern Orthodox Church. The word βρῶμα actually means any and all solid food, not simply animal flesh. With the sole exception of Matthew 14:15 (victuals), βρῶμα is translated as "meat" in the 1611 King James Bible, as in the earlier Tyndale (1536) and Geneva (1557-1560) Bibles, which is consistent with the usage of the term dating back to Old English. It is the first definition of the word given in the Shorter Oxford English Dictionary (6th ed.):
I 1 Food; nourishment for people or animals; esp. solid food, as opp. to drink. Now arch. & dial. OE
Many of those who follow the King James Bible allow themselves to be misled by imposing modern meanings on words that meant something different at the time the translation was completed. Meat is a relatively benign example (atonement is more insipid). Most, but still not all, understand that "meat" meant food in the King James Bible, as is reflected in succeeding Protestant translations:
... forbidding to marry, and commanding to abstain from foods ...
... who forbid marriage and require abstinence from foods ...
They forbid people to marry and order them to abstain from certain foods ...
The second point, having now correctly interpreted what the Greek means, is that Paul's objection in 1 Timothy 4:3 is not that people abstain from food (ἀπέχεσθαι βρωμάτων), but they do so for the wrong reason: namely that they reject food because they see it as evil and not as something good that is provided by God - a prevailing gnostic tendency of the time. Irenaeus writes on this in Against Heresies I.XXIV and elsewhere.
The proper understanding of 1 Timothy 4:1ff is explained by the Romanian Orthodox Elder Cleopas in The Truth of Our Faith (pp. 146-147 - a very good book; I highly recommend it):
Some are convinced that we should eat of all foods, at all times, without discretion. They say that we should cast off all the restrictions of the fast and make wide the road to the belly. However, we have a teaching from our Saviour Christ that the demons are not cast out except by prayer and fasting (Mat 17:21). His holy disciples and Apostles ministered to the Lord with fasting, as it is written: As they ministered to the Lord and fasted ... and again, And when they had fasted and prayed and laid their hands on them, they sent them away (Acts 13:1-4). The great Apostle Paul did not set aside the work of fasting even in the midst of his trials: in stripes, in imprisonments, in tumults, in labours, in sleeplessness, in fasting ... (2 Cor 6:5).
Thus, whom should Christians obey? Our Saviour Christ and His holy Apostles or he who subverts and distorts the meaning of Scripture? (2 Pet 3:16-17). The passage, which came to you in your thoughts [1 Tim 4:3-8], does not assert the abolition of fasting. From a similar misreading of this passage sprang the delusion of the ancient heretics, the so-called Gnostics. The Gnostics forbade marriage and the eating of meat. These prohibitions they kept not in short stretches of time, as we do during the periods of the fast, but they perpetually forbade marriage, so as not to propagate "matter", while meat they considered to be unclean.
This article from NPR touches the high points of the history of the question to fish or not to fish.
While the Catholic religion required fasting and using fish, the Anglicans also entered the fray after King Henry VIII (and Anne) started the Church of England. Fish became a political choice, but as the industry declined, Henry's son eventually reinstated fish laws.
There was, and still is, a certain darker side to the fish story with the bible warning about those who abandon the faith. One sign is ordering to abstain from meat. See 1 Timothy 4:1-11.
On a lighter note, the article ends with the McDonald's franchiser who invented the Fillet-O-Fish because his hamburgers weren't selling well on Fridays.
So, it would appear that the rumor of Vatican owned fisheries and not diversifying into the cattle industry are unfounded. Cut that line.